Use the flavour system to create individual css files. Only the default...

Use the flavour system to create individual css files. Only the default flavour (style / style.ini) will have the default styles for interwiki and filetypes. There are two new events:

 1. CSS_STYLES_INCLUDED to modify the list of stylesheet files for a certain mediatype or prevent the mediatype or styles within it. This event is fired for the interwiki and filetypes too, to prevent that they are included
    * The Data that is being sent to the event contains: list of files or the keyword DW_DEFAULT for the interwiki / filetypes; the mediatype, the flavour, if the data should be encapsulated by a default @media and the encapsualation prefix (e.g. @media screen)
 2. CSS_CACHE_USE to prevent the use of a cached version of the CSS, just like the JS event that already exists.