This adds a new read-only property selection
and selectionMode to TableView with the following features:

  selection.selectAll()
  selection.select(from, to)
  selection.deselect(from, to)
  selection.forEach(callback)
  selection.contains(index)
  selection.selectionChanged()
  selection.count

Change-Id: I3cbd433851354bb68a6075f0b31f46dab8c83fd4
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>