Issue 68 multiple menubars
Needs previous merge request !70 (merged) to be applied first.
Bugfix: to support multiple menu bars, the callback lambda in dtkWidgetsMenuBarContainer::switchToRoot() must not be static. Otherwise, the same callback for all menubars will only be instantiated once, and it will only capture the container instance of the first menu bar, and it will only work for first menu bar calling this function.
Fixes issue #68 (closed)