Isolate ISourceNode/ITypedElement stuff in ElementModel namespace

[ewoutkramer]

Should we make it so that ElementModel becomes "legacy"? That you don't need it if you are not using ITypedElement etc anymore? The CQL types are probably a problem here, but it would nicely separate out the non-poco stuff from the poco stuff.

[Kasdejong]

if we decide to do this, we should do this before Alpha 2. PocoNode is currently also in ElementModel due to its "close" relationship with ITypedElement-based "dynamic" navigation