Make ModelInspector a true registry/factory for ClassMappings

[ewoutkramer]

There will be multiple ways to generate ClassMappings:

• By hand/from a source code generator
• From a structuredefinition
• From the reflection info on the POCOs (current situation).

We need a new abstraction (or extend ModelInspector) to serve as a point where users can register such mappings or mapping factories. In any case, the current reflection code in ClassMapping needs to be moved somewhere, and we need to be able to have a way to add classmappings manually (as we've done in the PR for almostchristian). And then for the future, a way to add them from SDs.