window: Delegate model creation to CcApplication

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.

shell/cc-application.h

@@ -19,6 +19,8 @@
 
 #pragma once
 
+#include "cc-shell-model.h"
+
 #include <gtk/gtk.h>
 
 G_BEGIN_DECLS
@@ -29,4 +31,6 @@ G_DECLARE_FINAL_TYPE (CcApplication, cc_application, CC, APPLICATION, GtkApplica
 
 GtkApplication        *cc_application_new                    (void);
 
-G_END_DECLS
+CcShellModel          *cc_application_get_model              (CcApplication *self);
+
+G_END_DECLS