qt-config-gui: Implement simplified view for picking features

The detailed tree of configuration flags that qt-config-gui currently presents is not very friendly to approach. Many of the available choices also have a relatively small effect.

As a user it would be much better to see a more limited set of higher level choices with descriptions that you could make easier choices on. The current detailed tree would be the "Advanced" option for this.

The simplified choices would be either especially important flags or include/not include multiple actual feature flags with a single choice.