Fix left/right cursor keys in TextArea

Since commit 5dc805c86077e key events for left/right cursor
get eaten by the MenuBar. That means, the events shouldn't be accepted
if no menu is shown.

Change-Id: Icc249a3c4d96e19e8c902276938a2972cf1364d9
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>