Make the presentation compiler independent of a Scala project.
Remove status handler for missing Scala library, and instead use a marker resolver.

This removes the pop-up dialog for a missing Scala library when trying to instantiate
the presentation compiler. Instead, the error marker in the Problems View, which was
anyway created, now has a marker resolver (enabling QuickFix — Ctrl-1), just like
Scala Installations errors.

This makes it possible to remove the IScalaProject dependency from PresentationCompilerProxy,
which should be able to manage any presentation compiler (with or without an associated project).
Extract project-specific logic out of the PresentationCompiler proxy.

Also added a factory method in the API.
Decouple watchdog from compiler proxy through Pub/Sub.

Also fix some NPEs during tests.
Cleanup locate() and friends.

Signed-off-by: François Garillot <francois.garillot@typesafe.com>
Presentation compiler elements for ScalaDoc
Adding Presentation compiler tests, including inherited & param symbols
Completion Integration for ScalaDoc
Integrate headerForSymbol into API
Integration of scalaDoc with ScalaHover.

ScalaDoc is only obtained through F2.