Allow to set a default XDM schema in the Alloy configuration

[vivasd]

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

[stewartschillingsdi]

Just adding my +1 to this in case it matters.

[dancingcactus]

@stewartschillingsdi Oh it matters. Thanks for the vote.

[arobsco]

As I've alluded to on Slack before, this is very much needed 🙏🏻