Status area: add NetworkManager indicator

Adds an implementation of nm-applet in javascript. Uses the new
introspection from NetworkManager, and temporarily requires
nm-applet to be running for the secret service.
Features a renewed interface, with each device controllable through
a switch, which if toggled off disconnects, and if toggled on
connects to the most recently used valid connection. More esoteric
features like creation of ad-hoc networks have been moved to the
control center panel.

https://bugzilla.gnome.org/show_bug.cgi?id=621707