Don't serialize items when a source item exists higher than in the item tree
Finished to clean ETLayoutItem/Group model descriptions
Deprecated ETCompositeLayout class hierarchy and ETInspector (not included in the build anymore)

Note: Will be replaced later by a better alternative that plays well with CoreObject diff.
When a composite layout was set up, the content routing was breaking diffs targeting ETLayoutItemGroup.items, since the items were moved to a parent with another UUID.