Fix bug when closing Undo History with a keyboard shortcut

Closes #1423

The bug was happening because when the Undo History window was
closed using the keyboard shortcut, the message kCloseMessage
was never being sent to the window. This is probably not the
only cause, but making UndoHistoryCommand close the window instead
of just hiding it solves the problem.

src/app/commands/cmd_undo_history.cpp

@@ -246,7 +246,7 @@ void UndoHistoryCommand::onExecute(Context* ctx)
     g_window = new UndoHistoryWindow(ctx);
 
   if (g_window->isVisible())
-    g_window->setVisible(false);
+    g_window->closeWindow(nullptr);
   else
     g_window->openWindow();
 }