Better GIT support

[Gamer-XP]

When multiple people work on the same project - there are a lot of issues, even with external levels options.

1. UIDs. They may easily collapse when merging 2 commits, and there is no way to fix that but to manually change all uids used, and increase NextUID value in the project file itself

• When loading, you can, likely, check which UIDs is used by which asset type. Use (Type, UID) key instead of just UID to store the link internally. With this you can check if UIDs collide in a lot of cases (as long as types were different) and fix UIDs by incrementing them.
• Auto-increase NextUID to max(NextUID value, actual max uid value + 1)
• Try to go away from UIDs to GUIDs. Json will bloat a bit though.

2. Editing main project file may be tough to make right, but, at least, letting other people add new levels freely is very important point. As long as project file is not affected - can easily add new levels, then merge the project.

• Can make LDTK auto-import level files it finds in the project folder. If it's a new level - check if UID is free. If not - assign a new one
• Maybe, instead of auto-importing, can make it so you can drag-n-drop level file in LDTK window to import it