Signals: Port more uses of connect_compat

Those were problematic as they call a method of their parent class,
but callable_mp does not allow that unless it's public.

To solve it, we declare a local class that calls the parent class'
method, which now needs to be protected to be accessible in the
derived class.

editor/editor_dir_dialog.h

@@ -50,6 +50,7 @@ class EditorDirDialog : public ConfirmationDialog {
 	bool updating;
 
 	void _item_collapsed(Object *p_item);
+	void _item_activated();
 	void _update_dir(TreeItem *p_item, EditorFileSystemDirectory *p_dir, const String &p_select_path = String());
 
 	void _make_dir();