Add a naive method of retrieving Unity3D.DLLs project properties.
Use a better method of enumerating projects for Unity3D.DLLs settings.

Access the current MSBuild property on a given project, instead of
assuming that the property is stored in the user project.
Add functions for setting MSBuild properties and item metadata.
Complete the implementation of Update-Unity3DReferences.

Set HintPath on a Unity DLL reference if Unity3DUseReferencePath is
false, otherwise set the ReferencePath property in the user project.
Add a function to join the Unity managed path property to ReferencePath.