Add libinput_next_event_type()

Add libinput_next_event_type()

Returns the event type of the next event pending in the queue. For systems
that have the device init state separate from the actual event procesing
(read: xorg drivers) we need to be able to peek at the next event type to
check for the end of any initialization events (seat/device added) and the
beginning of actual device input events.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>