TextArea: fix word wrap layout

Since scrollbars affect the document/viewport size and vice versa, we
must allow the layout loop to recurse twice until the sizes stabilize.
Furthermore, avoid nasty content size binding loops (from ScrollView)
by providing a specialized Flickable item that TextArea has full
control over.

Task-number: QTBUG-30832
Change-Id: I74fe6c5017e75aa411fdbb64087afa18e1221ce9
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>