After introducing the autocleanup function to
CcPanel, it is now possible to remove a lot of
boilerplate code from the panels.
This commit ports CcKeyboardPanel to be a final
type, removing all the old boilerplate code in
the proccess.
https://bugzilla.gnome.org/show_bug.cgi?id=769063
This makes loading faster, with less I/O, avoids unnecessary
code duplication (around 1k lines shaved), and ensures that
all the panels link and work appropriately.
By the same token, it will stop external panels from being
created, and loaded.
https://bugzilla.gnome.org/show_bug.cgi?id=690036
