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

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.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/787