view: Don't reload playlist when clicked multiple times

If the user selected the current playlist displayed, don't reload the
songs to prevent items to be added multiple times to the model when
double-clicking playlists.
widgets: Fix styling of Add to Playlist dialog

- Lessen row height
- Hide action area
- Add margin around the list
- Add separator between each row

https://bugzilla.gnome.org/show_bug.cgi?id=724627
widgets: Select new playlist after editing is done

https://bugzilla.gnome.org/show_bug.cgi?id=724627
widgets: Dim "New Playlist" item in Add to Playlist dialog

https://bugzilla.gnome.org/show_bug.cgi?id=724627
player: Restart song instead of replay

Setting the scrollbar position to zero emits seeked signal and forces
MPRIS clients to pickup the position change when the song is restarted.