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-new-call-box.c

@@ -301,7 +301,6 @@ calls_new_call_box_init (CallsNewCallBox *self)
 static void
 dispose (GObject *object)
 {
-  GObjectClass *parent_class = g_type_class_peek (G_TYPE_OBJECT);
   CallsNewCallBox *self = CALLS_NEW_CALL_BOX (object);
 
   clear_dial_queue (self);
@@ -311,7 +310,7 @@ dispose (GObject *object)
       remove_origins (self);
     }
 
-  parent_class->dispose (object);
+  G_OBJECT_CLASS (calls_new_call_box_parent_class)->dispose (object);
 }