Add libinput_device_touch_get_touch_count()

Add libinput_device_touch_get_touch_count()

This makes it possible for callers to detect whether a touch device is
single or multitouch (or even check for things like dual-touch vs real
multi-touch) and adjust the interface accordingly.

Note that this is for touch devices only, not touchpads that are just pointer
devices.

https://bugs.freedesktop.org/show_bug.cgi?id=104867

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