Simplify ToggleActionMenu

* Remove the ImplicitDefaultAction intelligence, so ToggleActionMenu is
  not more than a KActionMenu with setDefaultAction().
* Instead, reset the default action when it gets removed from the menu().
  This is done by filtering QActionEvent from menu().
* Add an autotest for ToggleActionMenu.

This replaces prior efforts to fix problems in ToggleActionMenu
in !245 and !254, following the discussion on the virtual meeting
at 2021-02-26.

6b26a2b4 and 1786e6c9 have already ported PageView and
AnnotationActionHandler to the simplified interface.