Streamline code updating the registry.

- Remove repetive string additions.
- Use one instance of QSettings only.
- Remove unused variable typeLibVersion.

Task-number: QTBUG-42824
Change-Id: I058cd3f7d844b185b0a9b3d1384bde1fb2dd1881
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Add categorized logging for registration of Active X controls.

Setting the environment variable
QT_LOGGING_RULES=qt.activeqt.registration.debug=true
produces output about the registration.

Task-number: QTBUG-42824
Change-Id: I964e1eafd345babdf3581399192a7b5702dea5a4
Reviewed-by: Andy Shaw <andy.shaw@digia.com>