With the current approach, we get duplicate labels
in the accessible name: _Cancel Cancel. Change things
around to always set the labelled-by accessible relation
if we have a label, and not the label accessible property.
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
Set the accessible role for GtkLinkButton to button.
We don't use the 'link' role since ARIA says "if it
behaves like a button, use 'button'".
Update docs and add a test.
This changes should not be neccessary, since
GtkLinkButton derives from GtkButton, see #2965.
