popupMenu: Don't chain up vfuncs if the parent doesn't implement them

Some vfuncs like button_press_event, button_release_event and touch_event don't have handlers in the parent classes of PopupBaseMenuItem. So don't call those handlers and return the default Clutter.EVENT_PROPAGATE there.

This fixes a crash of the shell that happens when pressing a mouse button inside the system popup menu and releasing it above a slider like the volume slider again.