Copying files in VS doesn't preserve build-action

[ssrmm]

Whenever a file is copied to or from a Nemerle project, the "BuildAction" property is reset to "Content".

If both projects are Nemerle projects, the build action should be preserved. Ideally this would also happen when copying between different projects (e.g. .csproj and .nproj) if both projects support the build-action of the file.