Hardens isCheckingClasspath

Removing unwanted changes in ScalaClassPath constructor (merge error).
Harden Settings toggling
Switch most of Classpathmanagement internals to using ScalaVersion
rahter than Strings
remove deprecations/unused from sdt.debug
Avoid possible exception on lost ScalaInstallation
Remove duplicate class
Split Scala Installation Management
Moves BuildSuccess and its Listeners to Pub/Sub
Introduces api.ScalaProject

Moves most of functionality to API package for ScalaProject

Documents the API trait.