Basic Google Music Support

* Support for following operations: login, getting all tracks, getting url from track ID
* Settings page
* SQlite backend with search & global search support

Remove some duplicates in code, minor fixes

Fix Load all tracks method

All API Stuff moved to one class, some code refactoring

Playlist create, rename and delete features
Merge branch 'googlemusic' of github.com:blackjack/clementine into googlemusic