UI: Wipe undo/redo stack when switching scene collections

UI/undo-stack-obs.hpp

@@ -38,6 +38,7 @@ public:
 	void disable_undo_redo();
 
 	void release();
+	void clear();
 	void add_action(const QString &name, undo_redo_cb undo,
 			undo_redo_cb redo, std::string undo_data,
 			std::string redo_data, func d);