CDN widget set should be installed to local Maven cache

[Artur-]

Right now when you use CDN in fetch mode, the widget set is downloaded to the target folder. If you do mvn clean install, the widget set is re-downloaded every time. If you happen to do mvn clean install when you are offline, you are out of luck: Caused by: java.net.UnknownHostException: wsc.vaadin.com: nodename nor servname provided, or not known

If the compiled widget set would instead be downloaded as a Maven dependencies (to ~/.m2), it would work offline as long as you do not add/remove add-ons. A mvn clean install would also be much faster when you have a slow connection as it would not need to re-download the widget set every time