Automate build for nos-elements

The goal is to package and build the newly created nos-elements to be used in Connect, Studio and Marketplace.

We should create a top level folder at nos repository containing all the cross application elements (nos-header-bar and nos-styles for the moment), then automate the build using webpack. Webpack build should be triggered using nuxeo-connect-plugin in a very similar fashion to what happens in nuxeo-studio-ui at the moment.

Current nuxeo-studio-elements should depend on the nos-elements package. Additionally, nuxeo-designer-styles should be reviewed: common parts might be extracted to the top-level parent.