Make context reference counted

Make context reference counted

Instead of only allowing one owner keeping a libinput context alive,
make context reference counted, replacing libinput_destroy() with
libinput_unref() while adding another function libinput_ref().

Even though there might not be any current use cases, it doesn't mean we
should hard code this usage model in the API. The old behaviour can be
emulated by never calling libinput_ref() while replacing
libinput_destroy() with libinput_unref().

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