Let EditorPlugin have a version of _get_state()/_set_state() that works at the project level

[blackears]

I'm working on a docking window that I would like to store state between sessions. At the moment, EditorPlugin provides a way to save this per scene with _get_state()/_set_state() and per window with _get_window_layout()/_set_window_layout(). As far as I can tell, there is no way to save this per project. Since I am using my dock to display resources it would be more useful to have the state be persistent across an entire project rather than for each scene within the project.