Automatic and manual game data paths

[Lgt2x]

As explained in #534, game data is currently searched for in Base_directory only. Once we support loading game data from multiple locations, we should make the experience more user-friendly. For this, we could have 2 mechanisms at work:

• Automatic lookup: Use standard game installation paths depending on the OS, such as Steam or GoG library path, mounted volumes. If we find *.hog files there, automatically add these paths to lookup locations.
• Manual selection: As a fallback, prompt the user with a path selection window where a directory can be specified. Check this directory content and use it if the data is found. After some research I could not find a simple solution for a cross-platform directory selection window, so a simple SDL2 input window could be enough for our needs.

In both cases, the resulting path should be written in game settings so this procedure only happens once.

This would improve UX dramatically, to avoid the game closing without any output such as #532