NuGet UI to support Add/Remove/Update of PackageRefs

[rrelyea]

Relies on work from Balaji to "Provide Add/Remove/Update PackageRef APIs for CPS" (issue # coming)

[harikmenon]

Spec - https://github.com/NuGet/Home/wiki/ToolsReference-and-Multi-TFM-Support-in-NuGet-UI

[kzu]

Let's make sure to preserve the DevelopmentDependency flag from the referenced nuspec, so we don't hit this #520

[rrelyea]

UI is post RTM