Since we eventually want to add a system for changing the top panel
contents depending on the current state of the shell, let's use the
"session mode" feature for this, and add a mechanism for updating the
session mode at runtime. Add support for every key besides the two
functional keys, and make all the components update automatically when the
session mode is changed. Add a new lock-screen mode, and make the lock
screen change to this when locked.
https://bugzilla.gnome.org/show_bug.cgi?id=683156
The design has a combined volume-network-power indicator in the lock
screen, which when opened shows a volume slider. Implement it by abstracting
the volume menu into a PopupMenuSection, and by creating three StIcons
bound to the real ones.
https://bugzilla.gnome.org/show_bug.cgi?id=682540
