Make QTreeView::indexRowSizeHint virtual

QTreeView::indexRowSizeHint() is quite slow for trees with lots of columns, since it calls itemDelegate(index).sizeHint() for every index in the row.  The return value from indexRowSizeHint() is cached in QTreeViewPrivate::itemHeight(), but the initial call to indexRowSizeHint() can cause a noticeable delay when scrolling to uncached rows.  If this method were virtual, it would be possible to use subclass-specific implementations that could be much, much faster.

We're using PyQt to implement a tree view that typically has hundreds of columns.  We've already reimplemented drawRow() to speed up painting and data fetching, and we were able to get an additional ~20% frame rate improvement when scrolling to rows with uncached heights by reimplementing indexRowSizeHint().