WIP: Migrate SDL2 graphics backend to SDL3

[tobbi]

This is WIP and updates our backend code to SDL3.

TODO:

• Fix compiler errors.
• Continue migration
• Replace local SDL_TTF clone with package.
• Implement proper bidirectional font algorithm (read when a language is RTL and set the appropriate tags to get arabic font shaping for example.
• Check where the sdl2 -> sdl3 replacement was too eager (where a replacement from sdl2 to sdl3 happened without being required - e.g. in po files)
• Check where else the version number needs to be bumped from 2 to 3.

... and more (I can't think of any right now, but there's other needs as well)