Add a class method to CcPanel to get a GOptionGroup which will be added
to the main commandline parser. This gives panels the chance to have
commandline "--flags" in addition to the already available parameters.
This changes changes the way parameters are passed to panels: the first
entry in the GVariant array is always the a{sv} dictionary of
commandline flags, followed by the remaining free-form arguments.
https://bugzilla.gnome.org/show_bug.cgi?id=696054
By using a GVariant of type "av" we can potentially pass more structured
data to panels, which will become relevant with the ability to invoke
them by GAction-based DBus-activation introduced in the following patch.
https://bugzilla.gnome.org/show_bug.cgi?id=696054
This promotes better encapsulation and allows us to move
application logic out of main() and rename the confusingly
named control-center.c to main.c
https://bugzilla.gnome.org/show_bug.cgi?id=692174
