Cleanup QTreeView autotest:
- use range-based for loops
- use nullptr
- use member initialization
- use new signal/slot syntax
- use static invocations / replace with QTRY_foo() calls
- use override
- use QStyledItemDelegate
Change-Id: I0e2d023254ed9f6f5d94cebf4d4358351cc4c3e2
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>