tst_QTreeView: cleanup

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 <[email protected]>