v4: Static View Models

[marner2]

Here is a summary of the "Static View Models" feature being added to v4.

• [x] Split apart BindingData and DynamicViewModel/Binding concepts
  • [x] #449
  • [x] #450
  • [x] #467
  • [x] #468
  • [x] #469
  • [x] #472
  • [x] #474
  • [x] #484
• [x] Make so that types are more variant (use get/set instead of ref/mutable types)
  • [x] #454
  • [x] #459
  • [x] #475
• [ ] Group related functionality more closely together
  • [x] #451
  • [x] #453
  • [x] #460
  • [x] #461
  • [x] #495
  • [x] #497
  • [x] #498
  • [ ] #489
  • [ ] #506
• [ ] Add typed 'vm argument to BindingData so that the property can be statically typed
  • [x] #464
  • [x] #505
  • [x] #504
  • [x] #470
  • [ ] #512
  • [ ] #511
  • [ ] #513
  • [ ] #508
• [ ] Implement ability to make an elmish view model type with statically-typed properties
  • [ ] #496
  • [ ] #507

There are fundamentally two parts to this push. The first is to allow statically-defined view model types to work with Elmish, and the second is to allow the properties of those view model types to be statically typed.

From a discussion with @TysonMN about the advantages of statically typed properties:

When used in the context of "dot" notation in the XAML project, you will be able to get compile-time help for things like ViewModelProp.SubModel.Prop. If those were constrained to obj, you wouldn't get compile-time checking of those types. Also XAML will warn you about type issues (number vs string, etc)

In addition, allowing statically-defined view model types will enable users to call out these view model types by name. This will allow things like data templates with explicit DataTypes set.