In order to support folder-hierarchy within Virtual URI mappings config, VirtualURIManager should be ported to an AbstractRegistry.

• The registry is bound to configuration-sources upon module startup
  • allowing virtual URI mappings by file at the same time
• Existing VirtualURIManager goes to deprecation
• Beware: registries are currently populated upon startup of the "owning" module. This is different to when virtual URI mappings & commands are loaded now (after all modules are started).
  • we could also consider generally deferring population of registries in similar fashion, while we're at it.

Side notes:

• Virtual URI mappings could have their own app (outside of About app)
• App could become a subset of Definitions app?
• Can registry offer specific API to know which mapping fired? Would it be the right place?
  • just mentioning here but might as well be considered in a superseding ticket.

Original title: Allow subfolders/nodes in the observed 'virtualURIMapping' module nodes

When having a lot of virtualURIMappings registered in a module, it can be quite hard to keep the overview.
Being able to sort them in folders/nodes would improve that a lot (as in paragraphs/dialogs already possible).

I added a patch of this class, which does that. Code should be checked for core quality, was implemented very quickly.