Simplify the way ViewViewModelFactory uses MEF

[jcansdale]

• GitHub Extension for Visual Studio version: 2.3.7 (dev)

The ViewViewModelFactory class currently has dependencies on ICompositionService and IGitHubServiceProvider, which I'm not sure are necessary. It also uses AllowRecomposition = true, which I don't think is supported in Visual Studio's flavor of MEF.

I'd like to see if this can be simplified, so we don't confuse people looking at this class in future.

[jcansdale]

To remove the dependency on IGitHubServiceProvider, we would need to replace exports like this:

[Export(typeof(IMyViewModel))]

with something like:

[ExportViewModel(typeof(IMyViewModel))]

there would be a kind of symmetry with:

[ExportViewFor(typeof(IMyViewModel))]

The ExportViewModelAttribute would export IViewModel and allow a ExportFactory to be located using the defined type (IMyViewModel in the example above). We could then import the VM factories into ViewViewModelFactory like:

[ImportMany]
IEnumerable<ExportFactory<IViewModel, IViewModelMetadata>> Views { get; set; }

The CreateViewModel method would be similar to CreateView and not depend on IGitHubServiceProvider.

@grokys what are your thoughts on this?