Allow dragging from shader editor to material

[KoBeWi]

Closes godotengine/godot-proposals#13802

I ended up also doing some refactoring.

[Calinou]

Tested locally, it mostly works as expected. Code looks good to me.

Undo/redo isn't working very consistently though. It will work in simple scenarios (assign material then undo), but if you perform multiple drag-and-drops, the first undo operation will appear to do nothing, then the second undo operation will revert to the state before either of the materials were drag-and-dropped:

shader_bottom_panel_drag_and_drop.mp4

Note that you need to right-click the 3D editor viewport once after the second drag-and-drop so that it gets focus (otherwise, undo will affect the text in the shader editor).

[KoBeWi]

Does the undo issue also happen when you drag a file from FileSystem? If yes then it's not relevant for this PR.

[Calinou]

Does the undo issue also happen when you drag a file from FileSystem?

Yes; I guess it should be fixed in a separate PR then.