Include selectivity.css directly in the template style

As it seems there is no poper way to include external stylesheets inside an HTML import, see :

• https://github.com/TakayoshiKochi/deprecate-style-in-html-imports
• https://github.com/Polymer/polymer/issues/1712
  And it causes errors when integrating in GWT (needed for Studio Application Definition):

  Refused to apply style from 'http://localhost:8080/nuxeo/studio/elements/bower_components/nuxeo-ui-elements/widgets/selectivity/selectivity.css' because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled.
  

It seems the simplest solution is to directly copy the content of selectivity.css inside the <template><style> tag of nuxeo-selectivity.html