[WIP] Port the project to Godot 4

[qrrk]

I finally decided to bite the bullet and make a serious attempt to port Catapult to Godot 4. I already looked into it before and know that it will be a lot of work, but it will have to be done eventually.
Some things will probably have to be seriously rewritten, especially the more hacky parts related to theming and windowing.

[qrrk]

If anyone is curious, this is how it looks so far, after a couple of evenings of fixing.
It launches (barely, still with some errors), but many systems are completely broken.

[qrrk]

Must have

• Fix any immediate blocking problems (script parsing errors, etc.).
• Verify core functionality and fix if needed:
  • Game installation.
  • Game launching.
  • Handling of multiple game installations.
  • Mod installation and management.
  • Soundpack installation/deletion.
  • Font customization.
  • Backups.
• Fix localizations.
• Fix tab disabling method in Tabs.gd (or find an alternative).
• Fix stable/experimental selection UI.
• Fix appearance of custom icon buttons.
• Revisit OSExecWrapper and decide what to do with it.
• Fix invisible scroll bars.
• Fix visual issues in themes.
• Fix icon scaling.
• Fix font scaling.
• Fix any other issues with theme scaling.
• Playtest on Linux.

Nice to have

• Modernize code according to changes in Godot 4 (new getter/setter syntax, typed arrays, callables, etc.).
• Zipping and unzipping can now be done by Godot itself. There is no need to rely on external commands or exe's anymore.
• Use OS.create_process for game launching.

Extra ideas (can be done later)

• Centralize duplicated JSON loading code that exists in many places.
• Add a new theme based on Godot 4 editor theme.
• Create a watchdog for the game process that will track if the game is running.