Add a fallback for nuxeo diff translation keys

Nuxeo diff (now part of web ui by default) uses specific translation keys in order to display the schema properties.

That forces developers to duplicate their translation entries when there is no good reason to have a different label for a property shown on a diff compared to a layout. Convention is also a problem as it doesn't take the schema into account.

GOAL
provide a fallback: if the objectdiff key does not exist, use the one provided by the web ui convention (label.ui.schema.schemaPrefix.property)

=> This implies doing an analysis on how we could provide a backport to 10.10 (do we use the convention on master?)