Exposed the DispatcherWindow to the public API.

Added acquire() and get/setDispatcher() methods. Removed dispatcher(), but that shouldn't be a compatibility break because it wasn't public until now.
DispatcherWindow.acquire() is now per-application
Merge pull request #1033 from andrewkaufman/exposeDispatcherWindow

Exposing access to the DispatcherWindow for a given script.