TreeView rowDelegate styleData misses an index property

Currently an index property has been added to the itemDelegate in QTBUG-45187.
Probably this could be also usefull inside the rowDelegate. At least for uniformity.

In general this could be wrong, given that a single row matches multiple columns and so multiple indexes.
This is true in the old QtWidgets world. However TableView (and so TreeView too) dropped support for
models with multiple columns by allowing only a single column with multiple roles.

This decision implies that exposing an index property in the rowDelegate is still correct.