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
This value is highly sensitive to changes in the theme; we
don't want to have to tweak hardcoded integers in this test case
every time we rev gnome-themes-standard.
Future iterations could do more heuristic matching, but this
is enough for now.
https://bugzilla.gnome.org/show_bug.cgi?id=704747
