albumartcache: Make albumart fetching truly asynchronous

Despite spawning this into a separate thread, this is still processed
quasi-synchronously, delaying the whole insertion.

Instead, use GIO/GdkPixbuf asynchronous APIs to load pixbufs, being set
as a low priority idle, the actual pixbuf loading will be deferred until
the UI is shown. this also means the "..." loading icons will be visible.

https://bugzilla.gnome.org/show_bug.cgi?id=760033