Disentangle presentation compiler and Scala project

The goal of this PR is to allow instantiating a presentation compiler with different settings than those in a particular project. See individual commit messages for details.

This isn't a simple refactoring, there is one change in functionality (replacing a status handler for missing Scala library with a quick-fix via marker resolvers).
Yeah:
- I'm fine with not "Handler"-ing
- I'm more or less fine with deferring classpath checks (i.e. scala-library) to classpath management.

But in the case of a PC crash at start for e.g. our sbt/classpath problem, I'm not sure the error log (a view which is non-default in the Scala Perspective, I recently noticed — so it's much less noticeable than the Problems view) is enough.

It's just a `(new cpMarkerFactory(markerId)).create(myProject, IMarker.SEVERITY_ERROR, myMessage)` away ...

 I'll defer to your best judgement and LGTM this without it though.