Fix back/forward/up menus for KF6
The popup menu for back/forward/up on the toolbar crashes KCacheGrind when KF6 is used.
It seems the KToolBarPopupAction widget changed in KF6, and menu() is now null() by default, and we should use popupMenu() instead.
See also: https://api.kde.org/frameworks/kwidgetsaddons/html/classKToolBarPopupAction.html