- Use nullptr, override
- Move the constructor body to the implementation on grounds of
  * Avoid code bloat by inlined constructors
  * Avoid the ugly C-style cast from IDispatch to IUnknown
    since both are just forward-declared in the header

[ChangeLog][ActiveQt] dumpcpp now generates the constructor code
into the .cpp file.

Change-Id: I180f64c2b32793d64080ecb8c4246b44b6c0b615
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>