With this design, a single PointerInterface manages multiple wl_pointer
objects. This makes the API tidier and allows implementing things such as
keyboard grabs more easier.

In addition to that, the PointerInterface doesn't inject its own frame
events anymore. It's up to the compositor to decide when it has to be
sent. However, the PointerInterface may still send a frame event if the
pointer focus changes.

Besides re-writing the pointer interface, this change, unfortunately,
also affects the implementation of pointer-gestures and relative-pointer
protocols because previously they were coupled to individual instances
of PointerInterface.