Unify metadata for completion engine and xamlx

[ShadowDancer]

Atm XamlX reads metadata from reflection or cecil, and completion engine reads metadata from dnlib.

I think it would be reasonable to create shared metadata model for Xamlx and Completion engine, to reuse metadata for both validating xaml and completing. Also it would add third xaml type system, currently there are 2:

1. reading form current appdomain
2. reading from source files

and the proposed one:
3. reading from compiled assemblies