Returns the widgets for which this widget is the target of a mnemonic.
Typically, these widgets will be labels. See, for example, [method@Gtk.Label.set_mnemonic_widget].
The widgets in the list are not individually referenced. If you want to iterate through the list and perform actions involving callbacks that might destroy the widgets, you must call `g_list_foreach (result, (GFunc)g_object_ref, NULL)` first, and then unref all the widgets afterwards.
the list of mnemonic labels; free this list with g_list_free when you are done with it.