Cleaned and reorganized ETLayoutItem public API

Moved private methods to ETLayoutItem+Private
Fixed copying for non-shared styles
Removed remaining old ETCopier code in ETLayoutItem

Now recreate ETLayoutItem transient state correctly.

Renamed ETLayoutItem.scrollViewShown to 'scrollable' and made it persistent

Made ETLayoutItem.valueKey derived
Sort and filter item tree after deserialization
Added proper serialization support for ETLayoutItemGroup.source

Removed old COCopier code in ETLayoutItemGroup