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.

doc/classes/Object.xml

@@ -354,6 +354,12 @@
 				Returns [code]true[/code] if the object is allowed to translate messages with [method tr] and [method tr_n]. See also [method set_message_translation].
 			</description>
 		</method>
+		<method name="cancel_free">
+			<return type="void" />
+			<description>
+				If this method is called during [constant NOTIFICATION_PREDELETE], this object will reject being freed and will remain allocated. This is mostly an internal function used for error handling to avoid the user from freeing objects when they are not intended to.
+			</description>
+		</method>
 		<method name="connect">
 			<return type="int" enum="Error" />
 			<param index="0" name="signal" type="StringName" />