Use freezed's sealed Union Types for the state machines instead of enums

[TechnicJelle]

Instead of having enums with additional variables on the side that have to be accessed only when the enum is set correctly, use this: https://pub.dev/packages/freezed#union-types

This will reduce the likelihood of potentially broken program states.

Relevant files:

• https://github.com/TechnicJelle/BlueMapGUI/blob/main/lib/main_menu/projects/project_tile.dart
• https://github.com/TechnicJelle/BlueMapGUI/blob/main/lib/main_menu/settings/java/java_picker.dart

And maybe this can also replace

• https://github.com/TechnicJelle/BlueMapGUI/blob/main/lib/project_view/configs/models/base.dart
  but will have to see. It's quite an integral part of the program now, and it does work pretty well already.