Using GList is a bit lame, and makes the API more complicated to use
than necessary in the common case.
The only real use case for a GList is gtk_widget_add_mnemonic_label(),
and for that we can use the GValue-based API instead.
Fixes: #3343
Use the label accessible role for GtkLabel. ARIA has some
ominous wording about it going way, but while we have it,
GtkLabel is the obvious candidate for carrying it.
Update the documentation and add a test.
