Update menu UI before showing a popup menu in wxQt

Give the application code a possibility to disable or otherwise change
the menu items before the popup menu is shown, as in the other ports.

Closes https://github.com/wxWidgets/wxWidgets/pull/1532

src/qt/window.cpp

@@ -1035,9 +1035,10 @@ void wxWindowQt::DoSetToolTip( wxToolTip *tip )
 #if wxUSE_MENUS
 bool wxWindowQt::DoPopupMenu(wxMenu *menu, int x, int y)
 {
+    menu->UpdateUI();
     menu->GetHandle()->exec( GetHandle()->mapToGlobal( QPoint( x, y ) ) );
 
-    return ( true );
+    return true;
 }
 #endif // wxUSE_MENUS