TextArea: match TextEdit height with the viewport height

Interaction (such as selection and flicking) was inconsistent
between inside and outside the internal TextEdit. For example,
it was possible to clear the text selection by clicking outside
the text but within TextEdit. If one clicked outside TextEdit,
the text selection wasn't cleared.

Change-Id: Ie315c4ed9047967ccb61f1b2338ec5d60fbb2033
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>