TableView: Reset dragged header position if not moved

The user expects the "ghost" header to be aligned with the actual
column header when he starts dragging it.

Change-Id: Ifccf633403a72fe8448132fe6c349a85acba28d7
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
TableView: Make sure we get the content width with hidden header

The key is to get a headerRowDelegate of non-null dimensions.

Task-number: QTBUG-39393
Change-Id: I5ceadb07ebfc84142bbbfe98c5d61780e0314cf5
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>