Alpha 7 - project-wise export into a single texture

Now you can export the whole project into a single texture or Godot tileset. This is basis to make other exporters, namely for Godot4 and Tiled. The tileset is overwritten.

Changelog

• Created a project export dialog with 2 export options - texture and Godot3 tileset.
• Tiles in the project tree now save their subtree collapse state. By default all are collapsed.
• Navigation in the project tree now can be done by keyboard, added some hotkeys also.
• Fix: subtile offsets now work for offsets between the frames.
• Added new tile parameter: texture offset. It is usable when saving tiles bigger than the grid inth tilemap, for example for diagonal 256 tiles.
• Some smaller UI and logic fixes