library: use RhythmDBImportJob when adding to the database (bug #547174)

RhythmDBImportJob provides progress information, so we can display that
to the user, and allows cancellation (not exposed through the interface
yet).

RhythmDBImportJob requires all URIs to be added before it starts
importing, but rb_source_add_uri only gets them one at a time.  To work
around this, we use a short timeout to collect as many as possible.

In case a new job is started before an existing one finishes, we keep a
list of them so we can cancel all outstanding jobs on shutdown.  Only
the progress information from the most recently started job is
displayed.