tablet: Stop multiple proximity-out events from being reported

Because bad distance events still trigger calls to tablet_flush(),
tablet_flush() will see that the tablet is out of proximity and assume it's an
appropriate time to send a proximity-out event, even when we've already sent
one. This results in multiple proximity-out events being sent in a row instead
of just one.

Signed-off-by: Stephen Chandler Paul <thatslyude@gmail.com>
tablet: Replace tool-update with proximity-out

A proximity-in event is something we want, especially since the current drafted
wayland spec has a proximity-in event. Adding this also makes our events more
consistent. And since we can just report the current tool in use with
proximity-in events, we can get rid of the tool-update event.

Signed-off-by: Stephen Chandler Paul <thatslyude@gmail.com>
tablet: Fix warnings in test/tablet.c

Signed-off-by: Stephen Chandler Paul <thatslyude@gmail.com>