feat: say when the system runs another version

The installer reads the version the system states in its own file,
without running anything: batocera.version, recalbox.version (a release
only), RetroBat's system/version.info, VERSION_ID in os-release on Lakka
and EmuELEC. When it differs from the version the list was built for, a
line says so before the install and again with the result, and the new
version event carries it for the screen. An unknown version says
nothing, and the install never stops on it.

wiki/installer.md

@@ -270,6 +270,20 @@ file list is downloaded once and handed to each run.
    [Files you already had](#files-you-already-had).
 8. Copies into standalone emulator directories, only with `--standalone-copies`.
 
+The file list names the version of the platform it was built for (Batocera
+43.1, RetroBat 8.3.1). Where the system writes its own version in a file, the
+installer reads it, and when the two differ it says so before the install and
+again with the result: "This list was built for Batocera 43.1, this machine
+runs 41. Batocera's own BIOS check may report some files as wrong though they
+work." The install goes on. The files read are
+`/usr/share/batocera/batocera.version` on Batocera, `/recalbox/recalbox.version`
+on Recalbox (a release, not a branch build), `system\version.info` beside the
+`bios` folder of RetroBat, and `VERSION_ID` in `/etc/os-release` on Lakka and
+EmuELEC. Nothing is run to find it, and an unknown version says nothing:
+RetroArch writes its version in no file, KNULLI writes a codename, ROCKNIX the
+date of the build, and AmberELEC's prerelease tags are not matched to the version
+its image writes.
+
 Some cores read their system data below the frontend's save directory, not
 below the BIOS folder: Azahar's keys, shared font and NAND files, Citra's
 `sysdata`, Dolphin's `User` tree. The file list marks them `root: save`, and
@@ -1534,6 +1548,7 @@ other schemas under `api/v1/schemas/`.
 | `detect` | `os`, `os_name`, `platforms` | Once the platforms are known. `os_name` comes from `/etc/os-release`. `platforms` lists `{platform, name, bios_dir}`, and is empty when nothing supported was found |
 | `error` | `stage`, `reason`, `platform`, `message` | The file list could not be read (`stage` is `manifest`): `reason` is `network`, `not_found` or `invalid`, `clock` when the server's certificate was refused and the system clock reads a date before the installer was written, `no_ca` when the machine holds no list of trusted certificates (`SSL_CERT_FILE` names one), or `offline` when `--offline` forbids the request. A folder named with `--from` cannot be read (`stage` is `from`, `reason` `not_found`, `not_a_folder` for a file that is no ZIP, `unreadable`, or `system` for the root of the system, `/proc`, `/sys` or `/dev`). The BIOS folder sits under a folder this user cannot open (`stage` is `dest`, `reason` `unreadable`), and the run ends with code 1 before anything is written. The BIOS folder cannot be created (`stage` is `dest`, `reason` `not_writable`): a `summary` follows with what the run installed before it. Two systems are detected on one machine and no `--dest` names the folder (`stage` is `detect`, `reason` `conflict`). The platform named with `--platform` is found nowhere and no folder can be assumed for it: a card system with no card named, a portable emulator, Batocera or Recalbox named on a computer (their folder is the `bios` folder of their SHARE drive), or no usual folder on this system or one set aside behind a folder this user cannot open (`stage` is `detect`, `reason` `not_found`). A `--dest` refused as the card of the system named carries the same `stage` and `reason`: another system's card, a folder inside a card, a drive with none of the files its card always has, or a folder that does not exist. `message` says it in words, and is the only thing that tells these cases apart |
 | `scanned` | `folder`, `files`, `truncated`, `other_drives` | Each folder named with `--from`, once read and before `detect`. `files` counts the files found, `truncated` is true when the search stopped at 200,000 of them, and `other_drives` lists the folders below it on another drive, which were not entered |
+| `version` | `platform`, `list_version`, `installed_version`, `message` | Before `choices`, when the system here runs another version than the one the list was built for: `list_version` is the list's, `installed_version` the one the system's version file states, and `message` says it and what it means. Not written when either version is unknown. The run goes on |
 | `choices` | `platform`, `systems` | Before `checking`: the systems the run's list lets one choose among, sorted by id, `other` last for the files no system names. The run's other options apply (`--core`, `--region`, `--no-game-data`, the files left out for an unknown save directory), `--system` does not: these are the systems a screen lists to choose from, and the count a `--system` is out of |
 | `file` | `platform`, `dest`, `systems`, `size`, `state` | Each file of the list after the check. `state` is `present` (hash verified), `wrong_hash` or `missing` |
 | `system` | `platform`, `system`, `name`, `files`, `present`, `missing`, `wrong_hash`, `bytes_needed` | Each system, sorted by id, and with `--system` only the systems named. Files with no system count under `other`. `name` is the platform's own name for it |
@@ -1554,7 +1569,7 @@ other schemas under `api/v1/schemas/`.
 
 <!-- /generated: events -->
 
-Schema 1 can gain a kind (`found` came with `--from`, `declared` with `--declared-only`, `unsearched` with the search that goes on past a closed folder, `app` with `--add-app`, `data` with the size of the data trees), never a key of an
+Schema 1 can gain a kind (`found` came with `--from`, `declared` with `--declared-only`, `unsearched` with the search that goes on past a closed folder, `app` with `--add-app`, `data` with the size of the data trees, `version` with the list's version), never a key of an
 existing kind: a reader skips a kind it does not know. An option refused
 before the run starts (`install.py: error:`, code 2) writes
 no event at all. A program reads a missing `end` as that case, or as a