Handle base texture file extenal rename/deletion

[BeeOProblem]

Renaming a texture is likely better handled through checking Godot's resource re-import signal since the AtlasTexture will still have the stale AtlasSource name which causes the current mechanism for detecting renames to break.

What needs to happen:

• Check re-import
• If current source texture is gone de-select
• Update all extant AtlasTexture to reference the new one if that's possible

[BeeOProblem]

Also should check if rename within godot itself causes any problems.