Add a language option to the VR menu, and fix a crash switching to it

- New "Language" row on the Main Menu tab (English/French/German),
  backed by a new "general" group in questshock.config.json
- Fixes a device-reproduced crash: switching language used to call
  language_dealfunc(), whose render_run()/opanel_redraw() repaint
  assumes the game's own in-game options panel is on screen - crashes
  (SIGSEGV) when triggered from our independent VR overlay instead.
  Now calls language_change() directly, which is all that's actually
  needed to reload resources and refresh the inventory/MFD/side-icon UI
- Fixes a second crash: this data set ships frnstrng.res/gerstrng.res
  but not mfdfrn.res/mfdger.res, and language_change() closes the
  current resource files before discovering the new ones don't exist -
  leaving no valid resource file open and crashing on the next lookup.
  Now checks both files exist before ever switching, refusing the
  change (with an in-headset popup) instead of corrupting resource state
- extract_assets.sh can now also pull mfdfrn.res/mfdger.res out of a
  System Shock: Classic installer (which includes them, unlike Enhanced
  Edition) if one is present in res/assets/ - entirely optional
- Documented all of the above in README.md/CONFIG.md

CONFIG.md

@@ -29,7 +29,7 @@ startup, in addition to (and independent of) the engine's own `prefs.txt`.
     load and so applies every group in the file, including the
     restart-required ones.
 
-  (Both options currently documented below are live-appliable, so this
+  (Every option currently documented below is live-appliable, so this
   dialog won't actually appear yet - it's built in for options added in a
   future version that do need a restart.)
 - Unrecognized groups/keys are ignored (harmless if you're editing the file
@@ -48,6 +48,9 @@ startup, in addition to (and independent of) the engine's own `prefs.txt`.
   "video_options": {
     "renderer": "opengl",
     "resolution": "1024x768"
+  },
+  "general": {
+    "language": "english"
   }
 }
 ```
@@ -69,3 +72,25 @@ The game's internal rendering resolution.
 - **Type**: enum
 - **Possible values**: `"320x200"`, `"320x400"`, `"640x400"`, `"640x480"`, `"1024x768"`
 - **Restart required?**: No (live-appliable)
+
+## General (`general`)
+
+### `language`
+
+The game's UI/text language - both the game's own resource strings and this
+project's own menu/keyboard chrome text (title bar, buttons, etc.).
+
+- **Type**: enum
+- **Possible values**: `"english"`, `"french"`, `"german"`
+- **Restart required?**: No (live-appliable) - applies the same way the
+  game's own in-game options menu applies a language change (reloads the
+  language-specific resource files and refreshes the inventory/MFD/side-icon
+  UI immediately).
+- **Note**: French and German both need their own full resource pack
+  (`frnstrng.res`/`mfdfrn.res`, `gerstrng.res`/`mfdger.res` under
+  `res/data/`) - if any of those files aren't present, the switch is
+  refused (logged, `questshock.config.json` and the current language are
+  left unchanged) rather than leaving the game with no valid resource file
+  open. See README.md section 3 ("Game assets") for where `mfdfrn.res`/
+  `mfdger.res` come from - the GOG Enhanced Edition installer alone doesn't
+  include them.