This fix ensures that we can only add a TableViewColumn exactly
once. It also automatically expands the column with to the viewport
when only one column is in use.

In addition I have disabled dragging when columnCount == 1
as it was pointless.

Change-Id: Ief6011c3e58166907836bf55b0fa6643698192d2
Reviewed-by: Caroline Chao <caroline.chao@digia.com>