Create one PointerInterface for each pointer wl_resource

Makes PointerInterface more like other Interface classes wrapping
wl_resource. The most important change is the handling of the
focused surface. This is now kept in the SeatInterface and can also
be set if there is no PointerInterface for the client yet.

The unit tests had to be adjusted and some are also disabled as the
button events are not yet moved into SeatInterface.