WPF SDK should explicitly contribute WPF ProjectCapability

Currently the WPF SDK does not contribute the "WPF" ProjectCapability. It is contributed by [Microsoft.Managed.DesignTime.targets](https://github.com/dotnet/project-system/blob/master/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/DesignTimeTargets/Microsoft.Managed.DesignTime.targets) but that does not apply outside of VS.

The ProjectCapability should be explicitly contributed by the SDK, so that proper detection of project type can be done outside of VS.