Fix shape and alignment of playlist gear button

- Change shape of playlist gear button to square
- Change alignment of the menu to end

https://bugzilla.gnome.org/show_bug.cgi?id=724627
Add borders to selection toolbar

https://bugzilla.gnome.org/show_bug.cgi?id=724627
view: Prevent the sidebar to widen in selection mode

Decrease size of artist name column in the sidebar of Artists view when
selection mode is enabled, to give space for the checkboxes.

Renamed variable to make its intended use clearer.

https://bugzilla.gnome.org/show_bug.cgi?id=724627
view: Replace player bar with selection bar in selection mode

https://bugzilla.gnome.org/show_bug.cgi?id=724627