The Network panel is not really a deriverable type, and
since after 61d7abe795 we can use the
utility macros.
Thus, this commit removes all the boilerplate code and
turns CcNetworkPanel into a final class.
https://bugzilla.gnome.org/show_bug.cgi?id=785581
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