Summary: mainAppContext, isDisposed, and numAppContexts has beem made volatile.  mostRecentThreadAppContext has been rewritten using ThreadLocal.
Reviewed-by: art

Summary: Added synchronization to AppContext.getAppContexts()
Reviewed-by: art

Summary: AppContext.dispose() should be ready to get exceptions during disposal of toplevels.  Also now we mark windows peers as destroyed when native object has been destroyed.
Reviewed-by: art