consider nearby levels when evaluating auto-rules

[SquareMan]

Closes #716

This is a first pass on this feature. I tried to implement it as minimally invasive as possible, but as far as I can tell levels interacting with each other is a pretty new concept to ldtk so I wouldn't be surprised if you have a lot of feedback.

Note: I am using a "dual-grid" tileset system in this example. That is why the int grid is offset from the level bounds.
Gif:

Considerations:

• Undo/Redo: Changes to neighboring levels are not saved to any timeline, but neighboring level's auto-rules are revaluated whenever a level undo/redo event occurs. This seems "fine" since no actual tile placement will change in a neighboring level, just their auto-tile cache, but I might be missing something.
• Neighboring levels are invalidated to ensure that they will be marked as dirty and have any updated auto-tiles saved when the project is saved.
• Auto-tile rules are reevaluated when a level is moved

Now I'll be trying to actually use this patch for a while in my own project and see if I find anything missing :)