Broke out PlayWatchService and AssetsClassLoader to be shared

These files can be used in multiple contexts.  In particular, they will
be needed by the forth-coming forked-run support needed by Activator
based on sbt-server.

[Updated] Added play.sbtplugin.run.PlayWatchService back with deprecation warnings

* Also, improved stand-alone sbt scripted test setup
I guess we can address this when it comes - Scala 2.12 is a long way off anyway.