TableView: implement currentIndex support

Implement support for using tableView.selectionModel.currentIndex to set a current index in TableView (and TreeView). Specification:

• Let the delegate have a "required property bool isCurrentItem" that tableview keeps updated (isCurrentItem from ListView. But "current" would better match "selected")
• Be able to move the current index around in the view using the arrow keys. Consider implementing a way to disable this default behavior.