input: Keep per client pointer resources in their own structs

input: Keep per client pointer resources in their own structs

Keep all per client wl_pointer resources in a new struct called
'weston_pointer_client'. When focus changes, instead of moving a list
of resources between different lists, just change the focused pointer
client.

The intention with this is to make it easier to add wl_pointer
extensions that share the same focus as the corresponding wl_pointer.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>