widgets: Use artist name as playlist id for Artists view

This allows the Artists view to easily lookup if the running playlist is
from itself.

https://bugzilla.gnome.org/show_bug.cgi?id=705852
view: Reshow albums widget of artist if running playlist

In Artists view, reshow the widget for the albums of the artist if
available and it is the running playlist. If it is not the running
playlist, destroy it before creating a new one.

https://bugzilla.gnome.org/show_bug.cgi?id=705852