Project Settings

[TechnicJelle]

Details copied from #8:

Proposed project settings should go in the project directory itself.
I think I'll use a json file for it...
I'd like to use a HOCON file, just like BlueMap's own configs, but there's just no (good) HOCON library for Dart.

The only thing we need to store in the SharedPrefs is a list of directories.
Maybe I'll add a way to name projects separately from their folder names at some point.
But maybe also not. I kind of like that the folder name is the thing that actually calls the shots.

In v2.0, could possibly also store which configs have been advanced'ed.