Replace "property list<TableViewColumn> columns" with:
- addColumn(column)
- insertColumn(index, column)
- moveColumn(from, to)
- removeColumn(index)
- getColumn(index)

Exposing list<TableViewColumn> type of property in the public API was
problematic for several reasons. First of all, it limited the internal
implementation too much. Secondly, modifying the list programmatically
did not work as expected, and it also threw nasty warnings while
reordering the columns interactively.

Task-number: QTBUG-30319
Task-number: QTBUG-31028
Change-Id: I0039f7e4be2d6ee9303a4118bdf84146b6a96a05
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>