[Design] Rework `ViewDataDictionary` constructors to ensure `ModelMetadata` is ne...

...ver `null`

- `ViewDataDictionary<TModel>` constructors now pass `typeof(TModel)` to base constructors
- remove `ViewDataDictionary<TModel>.ModelMetadata` override
- add more doc comments

nits:
- change `RazorPageActivator` to pass current `ModelStateDictionary`
- change 2 other `ViewDataDictionary` copy constructor calls to avoid
  uselessly copying `Model` and related information from source
- make a few `ViewDataDictionary` properties get-only
I don't really object to referencing private fields in a copy constructor... I think that's the point.

Could you give an example of some code that's simplified by a clone method? IMO copy-constructor is a better logical fit because to me 'clone' implies a full deep copy that's exactly the same. All of your 'clone' methods but one aren't total copies.