Exposed props of `ViewModel` should be specifiable in `part` call akin to `morphic` style props

[linusha]

Describe the Feature This would allow us to do

part(LabeledCheckboxLight, {label: '1233'}).openInWorld()

instead of

part(LabeledCheckboxLight,{viewModel: {label: '1233'}}).openInWorld()

where label is exposed on the ViewModel of LabeledCheckboxLight.

[merryman]

I am generally in favor of this idea. I think the reason I was not considering it from the start is because I was worried about issues that may arise with reconciliation. Implementing this will require the following steps:

• [ ] Enhance the part call to further apply the exposed properties after the view model has been loaded. This can be achieved by traversing all properties in a second round that are not considered to be managed by the style policies themselves.
• [ ] Enhance the expression serializer such that it also honors the exposed props when serializing a morph in order to become a component definition etc. This may turn out to be somewhat tricky, since we need to develop a machanism that deals with non expr-serializable viewModel properties. Maybe skipping these for the time being is enough?