EditorPlugin scene_change_requested (or similar) signal

[Swarkin]

I'm making a plugin that modifies newly added Nodes in the scene tree on the SceneTree node_added and node_removed signals.

Whenever the user changes the active scene, all node removed signals get emitted but there is no way of knowing whether this was caused by the user or the engine.

My proposal is whenever the user requests a scene change to emit a signal before the SceneTree gets modified in any way to help plugin creators in the situation I described above. (Workarounds are welcome as well)

[Swarkin]

Created an issue: #9619