sessionMode: Allow changing the session mode at runtime

sessionMode: Allow changing the session mode at runtime

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