Give xowiki chat also some kind of nice skin and centralize css retrieval
TODO: something smarter can be done in order to retrieve the proper css, based e.g. on theme, chat skin (to be implemented) etc. To be discussed is some way to resolve resources based on the theme.
Rationale: this feature makes little sense without the front-end xowiki provides and makes chat implementation harder to understand, as components are scattered in different packages.