Multiple issues with handling external filesystem changes

[BeeOProblem]

Changes to file contents are not detected by Godot engine and therefore cannot be handled easily. Best option is likely to refresh states from the filesystem on the editor gaining focus in the OS if that's an event we can track. We'd still be hosed if updates are pulled in the background but that can just go into an FAQ or known issues list or something

The following scenarios should work though:

• User renames or deletes an AtlasTexture that's associated with the currently selected source
  • Including when that AtlasTexture is the one selected for modification in QuickAtlas
• User renames or deletes a source texture that's currently being used
• User renames anything relevant that isn't currently selected

[BeeOProblem]

This is handled decently WRT changes to the AtlasTexture instances but changes to the base texture aren't handled. That's been spun into its own issue #27