input: Rename weston_surface_activate to weston_seat_set_keyboard_focus

input: Rename weston_surface_activate to weston_seat_set_keyboard_focus

The name suggests that it activates surfaces, but the code says it
rather just assigns keyboard focus.  Rename it for clarity, and so the
original function name could be used for something more appropriate
later.  Switch order of parameters since keyboard focus is a property of
the seat.  Update all callers as appropriate.

Change was asked for by pq, May 26, 2016:

 "This should be called weston_seat_set_keyboard_focus(seat, surface).
 Keyboard focus is a property of the seat."

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>