TextArea: fix document margin handling

The text margin must be within the document, not in the containing
item. Otherwise it breaks user interaction such as text selection
over the margin area.

Change-Id: I048c07f9c4e44336de6fa3f7b74b0cc56f4d0d5e
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>