Feature: Rename filename to match class name

[petterek]

[masaeedu]

Ideally this would be implemented as a more general purpose "move type into matching file" refactoring, which would create a new file matching the name of the type and place the type there. In the degenerate case where no types are left in the original file, it can be deleted.

[dnperfors]

FYI, This is already part of Visual Studio 2017.