Refactor Node Processing

* Node processing works on the concept of process groups.
* A node group can be inherited, run on main thread, or a sub-thread.
* Groups can be ordered.
* Process priority is now present for physics.

This is the first steps towards implementing https://github.com/godotengine/godot-proposals/issues/6424.
No threading or thread guards exist yet in most of the scene code other than Node. That will have to be added later.

core/object/message_queue.cpp

@@ -433,6 +433,17 @@ bool CallQueue::is_flushing() const {
 	return flushing;
 }
 
+bool CallQueue::has_messages() const {
+	if (pages_used == 0) {
+		return false;
+	}
+	if (pages_used == 1 && page_messages[0] == 0) {
+		return false;
+	}
+
+	return true;
+}
+
 int CallQueue::get_max_buffer_usage() const {
 	return pages.size() * PAGE_SIZE_BYTES;
 }