a11y: Emit text-changed signals directly

a11y: Emit text-changed signals directly

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.