input: Move surface picking into the pointer grab focus callback

input: Move surface picking into the pointer grab focus callback

Currently the core input code does surface picking before calling into
the focus callback of the current grab.  Not all grabs need to pick a
surface however, so we're doing work we don't have to in those cases.

For example, the shell move and resize grabs don't need to pick and the
default grab in implicit grab mode doesn't either.

With this change, the pointer grab mechanism is now very simple:
the focus callback is called whenever the pointer may have a new focus,
the motion callback is called whenever the pointer moves and
the button callback whenever a button is pressed or released.