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>
TabViewStyle: add support for left and right corner items

Empty corner items may be used as padding, so TabViewStyle::left/rightCorner
effectively replaces TabViewStyle::left/rightPadding.

Change-Id: I01f3a1552c3395352e8953456d3956f4f8c29aed
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
ComboBox: Fix warning when model not ready

Task-number: QTBUG-31135

Change-Id: I0d9b0a3cf9d1036e6940e10944c711cdd23d858a
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>