External links in wiki pages should be displayed with an icon to mark them as external

This is what we have on xwiki.org.
For example: http://www.xwiki.org/xwiki/bin/view/Main/WebHome

We need to decide if we want this as a specific skin feature, or elsewhere. Maybe we need some General Themes as we have Color Themes. In any case the decision where to put it shouldn't block the implementation of this (for ex it can be put in the skin initially and moved to a common place later on).