Clean up grab implementation

Clean up grab implementation

The grab API is a relic of Clutter 0.6, and hasn't been through proper
vetting in a *long* time — mostly due to the fact that we don't really
like grabs, and point to the ::captured-event as a way to implement
"soft grabs" in toolkits and applications.

The implementation of full and device grabs uses weak references on
actors instead of using the ::destroy signal, which is meant exactly for
the case of releasing pointers to actors when they are disposed.

The API naming scheme is also fairly broken, especially for
device-related grabs.

Finally, keyboard device grabs are just not implemented.

We can, in one go, clean up this mess and deprecate a bunch of badly
named API by introducing generic device grab/ungrab methods on
ClutterInputDevice, and re-implement the current API on top of them.