LibGUI: Clear undo stack when opening a new document

Author: https://github.com/pakerwreah
Commit: 67537bfc80
Pull-request: https://github.com/SerenityOS/serenity/pull/6868
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibGUI/UndoStack.h

@@ -26,6 +26,8 @@ public:
 
     void finalize_current_combo();
 
+    void clear();
+
 private:
     struct UndoCommandsContainer {
         NonnullOwnPtrVector<Command> m_undo_vector;