Icons for template queries should be configurable

See [1] step 3 about the icons. Definition and usage of template queries is done by configuration. But the for the icon you need an artifact in your cms.was with a specific name. This should be improved. For example by making the hippostd:icon property work again?

[1] http://www.onehippo.org/library/concepts/document-types/control-the-allowed-document-types-in-a-folder.html