Allow to set a default XDM schema in the Alloy configuration

As a customer I want to set an XDM schema as my default schema for that Alloy instance so that I don't have to manually select my schema again when creating an XDM object data element.

Submitted by Eric M and Rudi S


Start by tracing how Alloy configuration is defined and how an XDM object data element currently selects its schema. Determine where a per-instance default could be represented and applied; done means creating an XDM object data element automatically uses the configured default schema without manual selection.

Written by the indexing model from the issue text.