VS Project Loader ignores Directory.Build.props

[CharliePoole]

Since the project loader only reads the project files, it has no knowledge of any defaults in a Directory.Build.props file. That's especially a problem if the <OutputPath> element is included in a .props file because the output assemblies can't be located.

This appears to be a long-standing part of the implementation, probably an error in the initial design, so I'm calling this a new feature.

Note that if we implement a fix for #25 first, this will probably not be needed.