Add limit to UndoRedo

[dcuny]

Currently, there's no way to limit the size of the UndoRedo stack.

For applications with a very large UndoRedo history, this can prove problematic.

Currently, the only way to control the UndoRedo stack is to check the size of the stack with UndoRedo::get_history_count(), and clear the entire stack with UndoRedo::clear_history().

However, this is an all-or-nothing sort of approach.

If UndoRedo::_pop_history_tail() were exposed, it would be simple enough to implement at the user level, although there seems to be some finessing of the maintaining the Redo actions on the stack that I don't understand, which is why I'm posting this under Discussion and not Proposal.

It looks like a routine to clip the history stack could be implemented by writing a variant of clear_history():

void UndoRedo::clip_history(bool p_increase_version, int undo_limit) {
	ERR_FAIL_COND(action_level > 0);
	_discard_redo();

	while (actions.size() > undo_limit) {
		_pop_history_tail();
	}

	if (p_increase_version) {
		version++;
		emit_signal(SNAME("version_changed"));
	}
}

A cleaner approach might be to add this code block to the end of UndoRedo::commit_action. This would check the size of the history automatically.

For this to happen, it would also require adding a stack_limit attribute to the class, where 0 would indicate no limit and the default behavior.

Since changing the constructor is likely to be disruptive to existing code, I'd also suggest adding a UndoRedo::set_stack_limit() method.