Allow developers to set an entire folder for copying/publishing

See: https://github.com/dotnet/roslyn-project-system/issues/1579.

Allow developers to set an entire folder for copying to output directory via Project Properties, or any other common metadata, setting it would result in:

```

PreserveNewest

```

This will be quite complex, because what metadata should be applied to the folder? It would need to inherit all the metadata from it's contents, and merged so that only common metadata is present.