At times it is desireable to hide actors from being picked even
with a mode of CLUTTER_PICK_ALL.
Currently we use a pattern of
clutter_actor_hide();
clutter_stage_get_actor_at_pos();
clutter_actor_show();
in these cases, which gets hideous if the actor we want to exclude
from the pick is located in another module.
A more elegant solution is to connect a handler to the ::pick signal,
which stops further emission.
Credit for the idea goes to Owen Taylor.
https://bugzilla.gnome.org/show_bug.cgi?id=634560
1c8955b34a