Add TabView::contentItem

Task-number: QTBUG-38425
Change-Id: I9e2b960e9b336145388371e5cee8a8a4cb1fa38e
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Add styleData.pressed for TabViewStyle::tab

Change-Id: I54a1cfa3fe3824da6b554df90c5a96fa96229b74
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>