Added missing calls to -renderAndInvalidateDisplay and -will/DidChangeForProperty: in ETFlowLayout

Fixed usesGrid ivar name to support persistency
Fixed TableExample warnings

Made ETUIItemIntegration.h public so it can be explicitly imported for methods such as -owningItem if the current widget backend is AppKit.
Added TestTableLayoutPersistency

Changed -[ETLayout setUp] to take an argument indicates whether we are deserializing

Added missing calls to -will/DidChangeValueForProperty: in ETTableLayout

Fixed deserialization code in ETTableLayout(CoreObject)

Fixed recurrent crash due to [self zone] in ETNibOwner, there is no such -zone method (was probably working randomly just by luck)