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-call-selector-item.c

@@ -152,12 +152,11 @@ calls_call_selector_item_init (CallsCallSelectorItem *self)
 static void
 dispose (GObject *object)
 {
-  GObjectClass *parent_class = g_type_class_peek (GTK_TYPE_EVENT_BOX);
   CallsCallSelectorItem *self = CALLS_CALL_SELECTOR_ITEM (object);
 
   g_clear_object (&self->display);
 
-  parent_class->dispose (object);
+  G_OBJECT_CLASS (calls_call_selector_item_parent_class)->dispose (object);
 }