album editor dialog : update album cover

New feature

This is a minimal implementation of an album editor dialog. One can only update the album cover by selecting a new file

Relevant art

https://raw.githubusercontent.com/gnome-design-team/gnome-mockups/master/music/edit-details-instant-apply-wire.png https://raw.githubusercontent.com/gnome-design-team/gnome-mockups/master/music/edit-details-wire.png

Related: !305 (closed)