Cleanup QTableWidget autotest:
- use range-based for loops where possible
- use nullptr
- use member initialization
- use new signal/slot syntax
- remove a lot of C-style casts
- use static invocations
- use override
- instantiate objects on stack instead of heap to avoid memleaks
Change-Id: I99ed144caab88d648d5ab987ce0963fbc6f1197d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>