Added TestLayoutPersistency

Made ETLayout.dropIndicator persistent

Removed copy code in ETLayout

Added aliasing case support to UKValidateLoadedObjects() test macro
Wrote persistency tests for ETPositionalLayout and ETFixedLayout

Added ETFixedLayout model description

Fixed mismatched ivar and property for ETFixedLayout.autoresizesItems

Added missing calls to -will/didChangeValueForProperty: in ETLayout, ETPositionalLayout and ETFixedLayout