clutter: Merge Cally inside Clutter

To do so, the first commits cleans things a little bit and then add a ActorClass.get_accessible_type vfunc that allows to get rid of the factories and make out-of tree accessible objects possible, basically a copy of StWidgetAccessibleClass.get_accessible_type.

Then move things inside Clutter namespace in separate commits while cleaning things a little bit without being too noisy. Marked as a draft because it needs a GNOME Shell PR and testing that no regression happened during the code reshuffling.

The changes here are indirectly motivated by moving ClutterText in the future to libst along with CoglPango

GNOME Shell PR: gnome-shell!3433 (merged)

The PR can be tested using https://gitlab.gnome.org/feaneron/elevado