Bind remaining theme properties to their respective classes

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.

scene/gui/graph_element.h

@@ -49,6 +49,10 @@ protected:
 
 	Vector2 position_offset;
 
+	struct ThemeCache {
+		Ref<Texture2D> resizer;
+	} theme_cache;
+
 #ifdef TOOLS_ENABLED
 	void _edit_set_position(const Point2 &p_position) override;
 #endif