Qt Designer: Fix clang-tidy warnings about class definitions

- Use ' = default' for trivial constructors/destructors
- replace virtual by override or add override where applicable
- Return *this from assignment operators of custom iterators
- Replace trivial constructors by member initialization for simple structs
- Add Q_DISABLE_COPY

Change-Id: I67fb07cb82ccba93a41d58a8a0782628c9c681d9
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>