Allow offline mode

[MFAshby]

Allow downloading tracks for offline playback (i.e. extra check box column on table 'download to play offline' or similar)
Allow searching downloaded content when offline.
Allow playing downloaded content when offline.

[MFAshby]

In progress:
You can right click & download tracks for offline playback
There is a switch to force offline mode in the SDK

Outstanding:
Search and play offline tracks.
Remove offline tracks

I'm going to do this by creating a second source RBDeezerOfflineSource, which will inherit most of it's functionality from RBBrowserSource!