JBIDE-13601 : fix errai deployment issues

This fix is 3-fold :
- tells GPE to generate the errai classes in a WTP friendly folder
- removes non-existing sources from build classpath as it provokes NPEs
  and prevents Dev Mode from running
- Refresh Dev Mode generated folders so these resources can be publishes
  automatically on any WTP server the app is deployed to.

 The Dev workflow is greatly simplified for the user :

 * Dev Mode :
   - deploy app on WTP server
   - run as > Web Application on external server (fill in url and select
     entry point)
   - click on dev mode link to open in browser

 * Slow mode :
   - click on Google > GWT Compile
   - wait
   - Run as > Run on server
   - open browser

Signed-off-by: Fred Bricon <fbricon@gmail.com>