Make GListModelInterface::get_item usable from GObject Introspection bindings

Language bindings have so far been unable to implement the GListModel
interface because the ::get_item virtual function returns a
non-bindable type (gpointer). The `gpointer` type gets translated into
`void` by G-I meaning that get_item() implementations can't return any
items.

We can set the return type of the get_item() vfunc explicitly to
GObject, which fixes the issue.

This patch also removes the existing (type GObject) annotation on
g_list_model_get_item(), which is necessary because if its return type
matches that of the get_item() vfunc, G-I connects the two and
propagates the 'skip' annotation from one to the other resulting in the
get_item() vfunc being hidden. There's no API break here because the
'skip' annotation makes g_list_model_get_item() invisible to G-I users
anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=787271

gio/glistmodel.c

@@ -95,6 +95,22 @@ G_DEFINE_INTERFACE (GListModel, g_list_model, G_TYPE_OBJECT)
  * Since: 2.44
  */
 
+/**
+ * GListModelInterface::get_item:
+ * @list: a #GListModel
+ * @position: the position of the item to fetch
+ *
+ * Get the item at @position. If @position is greater than the number of
+ * items in @list, %NULL is returned.
+ *
+ * %NULL is never returned for an index that is smaller than the length
+ * of the list.  See g_list_model_get_n_items().
+ *
+ * Returns: (type GObject) (transfer full) (nullable): the object at @position.
+ *
+ * Since: 2.44
+ */
+
 /**
  * GListModel:
  *
@@ -186,7 +202,7 @@ g_list_model_get_n_items (GListModel *list)
  * %NULL is never returned for an index that is smaller than the length
  * of the list.  See g_list_model_get_n_items().
  *
- * Returns: (transfer full) (nullable) (type GObject): the item at @position.
+ * Returns: (transfer full) (nullable): the item at @position.
  *
  * Since: 2.44
  */