Fix clang-tidy warnings about class definitions

- Use ' = default' for trivial constructors/destructors
- replace virtual by override or add override where applicable
- Replace trivial constructors by member initialization for simple structs
- Add Q_DISABLE_COPY or spell out deleted functions where only
  a copy constructor is implemented

Change-Id: I0b1610cb1425dd276a97df8179140c55f2e7b049
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>