Lots of work on the new background.

* Push rendering the background down to the MainWindow and make everything else transparent to make it shine through
* Add a few custom widget subclasses that do not render anything themselves
* Remove rendering of the backgronud from other widgets

This commit represents work in progress and there is still some serious wonkyness ( if that is not already a word, I hereby declare myself the inventor ) going on with the context view, and lots of dead code that I need to remove.