Call QAccessible::updateAccessibility when setText is called on QLabel

Call QAccessible::updateAccessibility when setText is called on QLabel

The method is called when the text of a label is changed and setAccessibleName has
not been called on the label, as the text of the label acts as the accessible name
of the label.

Merge-request: 1301
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@nokia.com>
(cherry picked from commit a1f2b68e97477440cf508e6d497eb5f5d9971971)

Change-Id: Ic10f75e72ac3faa84777c444177b287b720a1dc2
Reviewed-on: http://codereview.qt.nokia.com/3040
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@nokia.com>