Introduce skin extension plugins for webjar resources

In XWiki, we can use:

• $xwiki.ssx.use to enable stylesheets skin extensions ;
• $xwiki.jsx.use to enable javascript skin extensions ;
• $xwiki.ssfx.use to enable stylesheets skin extensions coming from the {/resources} folder ;
• $xwiki.jsfx.use to enable javascript skin extensions coming from the {/resources} folder.

But we have nothing for scripts and stylesheets coming from webjars. That's a shame.

Of course we can still use requirejs to load javascript afterwards, and @import url("$services.webjars.url('example', 'example.css')"); for CSS but it's less efficient (at least for CSS):
http://www.stevesouders.com/blog/2009/04/09/dont-use-import/

The best would be to have

• $xwiki.jswx.use() for javascript skin extensions coming from a webjar
• and $xwiki.sswx.use() for stylesheet skin extensions coming from a webjar.