This compositor-side object will single-handedly drive a window
drag operation. Currently, this largely copies meta_display_begin_grab_op
and meta_display_end_grab_op, except grabbing is done through a
ClutterGrab instead of direct meta_backend_grab_device() calls. This
also means that the switch from passive to active keyboard grabs is
handled differently.
Currently, this object is dormant. It requires moving more code from
other places to become a fully functional replacement.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2683>