[ Feature ] Add drag and drop image support

[iwubcode]

I'm going to assume this hasn't been done because there's a technical hurdle (maybe Godot doesn't support it?) but it'd be nice to drag and drop images into the paint (maybe material project, haven't used that?) to create an image node automatically.

[RodZill4]

It is implemented and used to work, but it's broken in Godot right now (tried to workaround the problem with very little success).