mtpdevice-plugin: add offline play notification

Submitted by Klaus Dorninger

Description

this patch includes 2 enhancements over the current mtpdevice plugin:

1.) it adds scrobbling based on libmtp's usecount field (this value unreliable so each track will only be scrobbled once). most of the code was simply taken from the ipod plugin

2.) device probing and db-loading is moved to a separate thread so the application doesn't get locked up

I haven't done extensive testing, but it has been working for me quite well for the last days