Make QToolButton::showMenu virtual

Sometimes it's necessary to update `QToolButton` menu dynamically, so `showMenu` is best place to do so. Current implementation doesn't allow to customize it, so `showMenu` should be wrapper over virtual function. See [discussion on forum|https://forum.qt.io/topic/127220/qtoolbutton-showmenu-as-wrapper-of-virtual-function] for background of this request.