Convert wl_input_device to wl_seat (and friends)

wl_input_device has been both renamed and split.  wl_seat is now a
virtual object representing a group of logically related input devices
with related focus.

It now only generates one event: to let clients know that it has new
capabilities.  It takes requests which hand back objects for the
wl_pointer, wl_keyboard and wl_touch interfaces it exposes which all
provide the old input interface, just under different names.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>