NXThemes: caching of /nxthemes-css/

dynamic styles (/nuxeo/nxthemes-css/) can be cached on the server and be invalidated using the theme manager's event system. currently they are generated on-the-fly for each request.

then they can be cached on the client by setting cache headers (the parameters are obtained from the nxthemes' 'application' nxruntime contribution)

then gzip compression can be turned on too if the browser supports it.