Adding StLabel a11y support

Adding StLabel a11y support

Right now is just redefine atk_object_get_name.

If someone wonders why not implement AtkText interface, or expose the
internal ClutterText, here a extract from AtkText doc:

"AtkText should be implemented by AtkObjects on behalf of widgets that
have text content which is either attributed or otherwise
non-trivial. AtkObjects whose text content is simple, unattributed,
and very brief may expose that content via atk_object_get_name
instead;"

StLabel is attributed, but is still simple and brief. In the same way
the atk_object_get_name redefinition is required, so this patch is the
first step. We can implement AtkText in the future.

https://bugzilla.gnome.org/show_bug.cgi?id=626658