Split xstyle into separate packages

[dylans]

We've received several requests to consider splitting xstyle up into a few smaller packages. The most common requests for separation are:

• AMD CSS loader plugin/build tool
• Some of the CSS utilities that are useful on their own

[kriszyp]

I have been doing some thinking about this, but I wasn't sure of the best way to go about this. Here is one way:

• new package (might call it "cassius") - CSS loading, build tool, utilities, extension parser (this is used by the build tool)
• xstyle - extensions (expression handling and base definitions)

This seems like it could be a little painful for some of the dependent packages like dgrid, that would have to point to a new package, and would make older dgrid versions incompatible with the latest xstyle, unless I kept some redirect modules in place (particularly xstyle/css). I could make the xstyle package contain the basic CSS tools and the extensions could go somewhere else, but than the naming would be kind of nonsensical.

[steveoh]

This looks like a very cool idea. It fits how we only use the cassius part of the xstyle package currently.