6645885: small refactoring for XContentWindow
Summary: move createContent() method from XDecoratedPeer to XContentWindow, so only XContentWindow keep information about the way we position it.
Reviewed-by: anthony
6636370: minor corrections and simplification of code in AppContext
Summary: mainAppContext, isDisposed, and numAppContexts has beem made volatile. mostRecentThreadAppContext has been rewritten using ThreadLocal.
Reviewed-by: art