Deskbar: fix showing an empty leaf menu, #3105.

* NeedsToRebuild() didn't actually do anything, as this
  returns whether it needs to be rebuilt or not.
* Replaced RemoveItems() with ForceRebuild(), which should
  already delete the menu items, whilst additionally
  letting the leaf menu know that it needs to rebuild itself.

This bug was reproducible 100%; however, with this fix, there
is now a rare race condition where clicking between an app menu
and the leaf menu will result in both menus showing concurrently.

src/apps/deskbar/BarWindow.cpp

@@ -136,7 +136,6 @@ TBarWindow::MenusBeginning()
 		return;
 	}
 
-	sDeskbarMenu->NeedsToRebuild();
 	sDeskbarMenu->ResetTargets();
 
 	fShowingMenu = true;
@@ -151,8 +150,7 @@ TBarWindow::MenusEnded()
 	BWindow::MenusEnded();
 
 	if (sDeskbarMenu->LockLooper()) {
-		// TODO: is this ok?
-		sDeskbarMenu->RemoveItems(0, sDeskbarMenu->CountItems(), true);
+		sDeskbarMenu->ForceRebuild();
 		sDeskbarMenu->UnlockLooper();
 	}
 }