DNN-5337 - Select an icon set

DNN 6 introduced the iconcontroller. I think the original intention with this was to have a flexible solution for using the icons but in the actual source code you only have the possibility to choose an icon from the actual "Sigma" collection. It would be nice to have a setting on portal level to select an Icon set. This could be another subfolder located in the icon folder beneath the Sigma folder containing all the needed icons. This could be even a great opportunity for third Party vendors to provide icon sets in the store. A github pull will follow.