"New Project" action etc should avoid mixing server and client code

Projects with mixed client/server classpaths are helpful for tiny demos, but can cause many irritating surprises for large applications as the classpath grows. Additionally, with J2CL it will not make sense at all to mix these classpaths.

A decent analogy would be writing an Android application with the server code in the same source directory and classpath.

As such, any method to enable the GWT facet for a project should at least make it difficult to do so for an existing server project, unless the user acknowledges the risks.