shell: add a autocleanup function to CcPanel

CcPanel uses the old boilerplate code from GLib,
which does not set an autocleanup function.

The lack of a cleanup function implies that panels
cannot use G_DECLARE_{FINAL,DERIVABLE}_TYPE, making
the code stick to the old boilerplate.

This patch adds a cleanup function to CcPanel. It doesn't
move CcPanel to G_DECLARE_DERIVABLE_TYPE() because it'd
break the CcPanel's subclasses.

https://bugzilla.gnome.org/show_bug.cgi?id=769063