TaskbarButton::overlayAccessibleDescription

Unify the following properties to "overlayAccessibleDescription":
- QWinTaskbarButton::overlayIconAccessibilityDescription and
- QQuickWinTaskbarButton::iconDescription.

"overlayAccessibleDescription" is nice pair for "overlayIcon" and
more importantly, consistent with QWidget::accessibleDescription.

Change-Id: Id5c3afb1c9db6881876983ce482d622c90db3f2d
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>