Added +allowsWebWorker prop for disabling the Web Worker in browser mode
Make CPLayoutConstraint immutable, like cassowary constraints.
Updated CPLayoutConstraintTest. The differents steps in the autolayout process are now more explicit.
-layoutWithcallback:f : optional callback after window layout (private method). TODO: use a CPOperationQueue for WebWorker out ?
-invalidateIntrinsicContentSize : do nothing if the window is not autolayouted.