Refactor

Hi Alexander,

This is a pretty big rework of the QtWaitingSpinner class.  You will notice that the core functionality remains largely the same and that most of the diff revolves around syntactic and stylistic changes.

Having said that, there is a fundamental change to the way the widget is constructed and how it behaves.  You will notice that I have replaced the constructor with one that allows the user to set the widget's modality as well as whether or not it should centre itself on its parent.  "Under the hood" the widget is now being created with Qt::Dialog and Qt::FramlessWindowHint flags and these constructor parameters allow the user to create, e.g. an application modal widget that centres itself on its parent widget.

If you are interested in the branch and have any questions, please do not hesitate to contact me.

Once again thank you for sharing this widget, it is a great piece of work!

Regards,
William.