Attach a stand alone ViewModel to a framework element

[mauroservienti]

The idea is to be able to allow the end user to do something like:

<SomeWpfControl behaviors:ViewModel.OfType="ViewModelType" />

Currently the only way to achieve the above is to expose an instance of the ViewModel from the current "parent" ViewModel, but it is not always what we really want to do, or another option is to leverage the entire UI composition framework.

Linked to: RadicalFx/Radical#113