Revert back to single threaded rendering for now, since my approach was wrong.

I have in fact learned how to implement multithreaded OpenGL rendering with Qt properly, but it adds considerable complexity and possibly fragility. Hence I have decided to re-evaluate multithreading at a later point, after the widget implementation itself is finished.

For reference, here are two articles that explain multithreaded rendering:
<a href="http://doc.trolltech.com/qq/qq06-glimpsing.html">http://doc.trolltech.com/qq/qq06-glimpsing.html</a>;
<a href="http://apsy.gse.uni-magdeburg.de/main/index.psp?sec=1&page=hanke/threadedcube&lang=de">http://apsy.gse.uni-magdeburg.de/main/index.psp?sec=1&page=hanke/threadedcube&lang=de</a>; (German)