This commit makes the label accessible implement AtkHypertext,
which returns a AtkHyperlink object for each link in the text.
At the same time, add AtkHyperlinkImpl objects as children
to the label accessible.
Also some private API to indicate that links have changed, and
call that from GtkLabel when needed.
Adjust expected output of the affected a11y tests.
https://bugzilla.gnome.org/show_bug.cgi?id=721410https://bugzilla.gnome.org/show_bug.cgi?id=721421
When setting new text on the label, the text-changed::delete signal
needs to be emitted before deleting the text (so that atk-bridge can
query the old text) while the text-changed::insert event needs to happen
afterwards (for the same reason). The old code using the notify signal
was only emitted after changing the text.
