feat: count game data apart from BIOS files

The installer knew BIOS files from game data and no count showed it.
The plan, the line menu and the result count them apart for a list that
marks game data, the plan gets a game data row and the menu a g key,
each toggling it in one press, and one sentence says what game data is.

wiki/first-install.md

@@ -92,6 +92,9 @@ The next screen counts missing files and free space.
 Press Enter on **Install**. **Choose systems** keeps the consoles you
 play.
 
+**Game data** is what a game reads, of no use to other cores. The count
+shows it apart, and its row leaves it out.
+
 **Old card?** In a card reader, choose **Copy the BIOS files from your old
 card or a folder**, then the card.
 
@@ -166,7 +169,8 @@ You cannot write there. Do what the message says.
 
 ### The card is full
 
-Nothing was written. Free some space, or choose another folder.
+Nothing was written. Free some space on the card. On a computer, you can
+also choose another folder.
 
 ### The game still does not start
 

wiki/installer.md

@@ -607,15 +607,24 @@ the full screen cannot open (a terminal smaller than 40x12, `TERM=dumb`), the
 same choice comes as questions after detection:
 
 ```
-RetroArch: 8069 files, 6.5 GB.
+RetroArch: 8348 files, 7.1 GB.
+  3259 BIOS files (4.4 GB) and 5089 game data files (2.7 GB).
   BIOS folder: /home/me/.config/retroarch/system
   Enter) install everything
   i) only what the installed cores read: 100 files, 44.2 MB
   c) choose systems, cores and regions
   f) copy from a folder or an old card
+  g) leave out the 5089 game data files (2.7 GB), game data: files a game or a game engine reads, of no use to the other cores
   q) quit
 ```
 
+A list that marks its game data counts it apart from the BIOS files, on the
+plan, in this menu and on the result (`Installed: 3259 BIOS files and 5089 game
+data files.`). `g` leaves it out, as `--no-game-data` does, and counts again
+(`g` then keeps it). The plan screen has the same choice as a row of its own,
+**Game data (5089 files, 2.7 GB): kept, leave out**, one press toggling it,
+and Options keeps its **Game data** row too.
+
 `i` appears for RetroArch, Lakka and RetroPie when cores are found installed,
 and the line above is a RetroArch with Beetle PSX HW, Snes9x, mGBA, Genesis Plus GX
 and FCEUmm.