volume: Implement new volume menu design

This is a part of the new system status design, see
https://wiki.gnome.org/GnomeShell/Design/Guidelines/SystemStatus/
for design details.

Since the designs require that we have a custom layout for the slider
item, this means that the PopupSliderMenuItem is unused, so remove it
as well.

https://bugzilla.gnome.org/show_bug.cgi?id=704368