TableView: implement keyboard navigation

Be able to select and navigate around currentIndex in TableView using the mouse or the arrow keys on the keyboard. Press "enter" to focus, and hold down shift + arrow keys to start a selection. Implementing required property "isCurrent" in the delegate.

TableView already supports assigning it a ItemSelectionModel. This model already contains a currentIndex property, which TableView should manipulate the same way it manipulates selections.