TableViewStyle: Fix and document styleData.hasActiveFocus property

The property has been present since 5.1 for rowDelegate and missing for
itemDelegate. Also, the property was incorrectly bound to the table
having active focus instead of the row.

Change-Id: I43a93975dffd849b37cfee6363343f8aef57d11b
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>