This commit ports CcApplication to use the modern
code practices and utilities, removing the boilerplate
macros. Namely, this commit introduces:
* G_DECLARE_FINAL_TYPE
* Remove unnecessary private field
* Unifies parameter layout
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