Huion/Gaomon handler needs to send proximity out event

[kurikaesu]

Currently the proximity out event is not sent (without any adverse effects that I can tell right now) however we really should.

What needs to be done is to create or update an existing timer after each digitizer event that will eventually fire once no new events are sent through over a short period of time. The callback would then call the handlePenLeftProximity() method in order to send the appropriate message.