Move custom editor to bevy_editor_plz

[janhohenheim]

After some initial trying out stuff: I think the way to go is to make SceneEditorState the EditorWindow::State and let the editor insert a new resource like CumtomEditorOutput, which contains the options the user just clicked on. Other systems can then properly dispatch the events, etc. We cannot do this directly in the editor, as doing so would require multiple mutable access on World.