This adds binds for GraphEdit/GraphElement/GraphNode, which were
skipped before due to a rework. This also adds binds for Window,
which was skipped before due to a complicated code organization.
Also adds theme cache entries/direct cache access to a few places
that previously missed it. Some theme properties are now exposed
to other classes via friendships or public getters for convenience.
This removes all string-based theme access from scene/ classes.
