Fix hovered link URI not returned by gtk_label_get_current_uri

Fix hovered link URI not returned by gtk_label_get_current_uri

This is needed for a query-tooltip handler, as mentioned in the
documentation, when there has been a hover timeout.

Maintain the previous behaviour when the link is clicked and follow the
existing documentation regarding selectable labels.

A notify::cursor handler can now also be used to retrieve the URI of the
link under the cursor.