Implicit transitions had a referenced taken while emitting the
completion signals, but said reference would only be released if it was
had remove-on-complete set to TRUE.

Change this to instead remove the 'is_implicit' state and mark all
implicit transitions as remove-on-complete. This fixes a
ClutterPropertyTransition leak in gnome-shell triggered by e.g. showing
/ hiding menus.

Fixes: gnome-shell#1740

!828