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/grid_container.cpp

@@ -259,6 +259,10 @@ int GridContainer::get_columns() const {
 	return columns;
 }
 
+int GridContainer::get_h_separation() const {
+	return theme_cache.h_separation;
+}
+
 void GridContainer::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_columns", "columns"), &GridContainer::set_columns);
 	ClassDB::bind_method(D_METHOD("get_columns"), &GridContainer::get_columns);