Using reference instead of copying when adding a catalog item to a page

For referencing catalog items we need to come up with a proper unique indentifier across all catalog items, across catalog groups, and (potentially) even across channels.

This also is critical for the SPA developers which need to 'map' a specific component (catalog item type) to their implemention logic.
Currently for SPA support catalog items are using an optional (and unconstrained) hst:label string property for that purpose, but that is very brittle and not 'stable' (modifiable).

So we probably can/should replace (drop) the hst:label property with something else which is and remains unique across the life-cycle of a component (e.g. defined at creation time only). Most practical may be enforcing a unique catalog item name (node name?) independent of the catalog group and even channel.
The catalog group can and probably is best seen as 'just' a grouping characteristic.

Alternatively we could enforce a path identifier (with more strict path element (node name) constraints, e.g. no spaces or funky characters) combining the <channel>/<group>/<item> elements. But that may become more cumbersome for SPA developers. This needs to be further discussed and aligned with SPA frontend development (e.g. Puma team).