Add CssMediaQuery abstraction

[gcotelli]

Currently, CSS media queries are embedded in media query rules, but they can be abstracted away so they became useful in other contexts. For example, a proper media query abstraction can be used as a parameter in many others IETF contexts like HTTP Link Headers.

The proposal is to have something like CssMediaQuery containing the media type and a list of media query expressions. It can be extracted by looking at CssMediaQueryExpression , CssMediaQueryRule and CssMediaQueryRuleBuilder.