Use correct parent class when chaining up overridden functions

How `g_type_class_peek ()` was used it didn't return the correct parent
class in most cases.
G_DEFINE_TYPE macro creates a pointer we can use to get the parent
class `n_p_parent_class`.
Because we didn't use the correct parent class the object initialisation
wasn't fully completed for some GtkWidgets.
See https://developer.gnome.org/gobject/stable/chapter-gobject.html#gobject-instantiation
for more information.

This commit makes use of the `n_p_parent_class pointer` created for this
specific use case where ever possible.

Fixes: https://source.puri.sm/Librem5/calls/issues/118

src/calls-application.c

@@ -423,7 +423,6 @@ app_open (GApplication  *application,
 static void
 constructed (GObject *object)
 {
-  GObjectClass *parent_class = g_type_class_peek (GTK_TYPE_APPLICATION);
   CallsApplication *self = CALLS_APPLICATION (object);
   GSimpleActionGroup *action_group;
 
@@ -432,7 +431,7 @@ constructed (GObject *object)
                                    actions, G_N_ELEMENTS (actions), self);
   g_object_unref (action_group);
 
-  parent_class->constructed (object);
+  G_OBJECT_CLASS (calls_application_parent_class)->constructed (object);
 }