Modularize continuations plugin.

The continuations plugin and library will still ship with 2.11 (albeit unsupported).
They now reside at https://github.com/scala/scala-continuations.
Modularize the swing library.

The Scala Swing library will still ship with 2.11 (albeit unsupported).
It now resides at https://github.com/scala/scala-swing.
Merge pull request #3221 from adriaanm/modularize-cps

Final library modularization for 2.11