Multi-project support

[TechnicJelle]

In SharedPrefs, store list of projects.

{
  "path": "/home/technicjelle/bmgui-projs/my-cool-world/",
  "mc-version": "auto",
  "mods": "/home/technicjelle/.minecraft/mods/"
  "advanced-configs": [
    "core.conf",
    "maps/my-fancy-overworld.conf"
  ]
}

Similar to IntelliJ.

On startup, show all projects.
Button to start new one.

You can close the project to go back to the projects screen.

If only one project exists, always open that one.
You can start a new project from the projects screen.

This should also make it so you can easily have multiple projects open at the same time.

[TechnicJelle]

Drew up some designs for the main menu:

[TechnicJelle]

These proposed options 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.

[TechnicJelle]

Added the multi-project support in many commits: 4b5af29...0c22346

The project settings will be moved to a new issue and tracked there. #29