In order be able to modify panel information statically,
we need to have access to the CcShellModel from static
functions. CcApplication, thus, is a better place for the
model to live, since we can access it outside any scope
using g_application_get_default().
It also makes sense from the modeling point of view, since
the model is not tied to the shell anymore.
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
