ICONTHEME: add a way to store a selected icontheme in the users preferences.
- add the 'icontheme' setting
- crawl all icon themes, remember it.
- check which theme was selected (if) or choose a default theme.
- add template token ICONTHEME to output the prefered icontheme or the default
- add iterator PREF:VALID:THEME to output a list of available icon themes
- add selector to the users preferences to choose the available icontheme.