Introduce a proper type for dynamic Skin resources

Right now, the URL for dynamic skin resources is:

http://localhost:8080/xwiki/bin/skin/skins/colibri/style.css?skin=colibri&colorTheme=xwiki%3AFlamingoThemes.Charcoal

However it's not normal that it's considered as an Entity Resource when it's pointing to an Entity. We should introduce a new type ("skin" for example - Note that static resources should use something like "resource" IMO) and instead have URLs like:

http://localhost:8080/xwiki/skin/colibri/style.css?skin=colibri&colorTheme=xwiki%3AFlamingoThemes.Charcoal