Fix not being able to set Node process priority in certain cases

[jsjtxietian]

Should fix #82339

I guess we can hide the process priority UI or just set the value without touching the thread_group part when _is_any_processing is false. This pr goes with the second option.

process_priority is only used in ComparatorWithPriority now so I guess this commit is safe to some extent. I would appreciate any feedback or alternative suggestions.

[RandomShaper]

Looks good. The key thing is that items affecting node thread groups (namely, processing on/off and priority) can't be changed if the node is already in one group (i.e., processing is on). The current code was mistakenly preventing a legal change.

That said, for consistency I'd suggest the same pattern as in set_physics_process(); such as:

	if (_is_any_processing()) {
		_remove_from_process_thread_group();
	}

	data.process_priority = p_priority;

	if (_is_any_processing()) {
		_add_to_process_thread_group();
	}

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.1.3.