popupMenu.js - Recalculate separator widgets when the allocation of
the menu changes.

Some popup menu actions that don't close the menu upon triggering can
leave the menu in an inconsistent state.  This adds a listener to the
menu's actor, any time a menu item is removed/hidden/shown/added, the
separator widget visibilities are recalculated to make sure no two
separators appear visible adjacent to one another.