Fix for FilePath mapping between Unix and Win

Hi Jonathan,

I tried to debug an app running on Win32 with VIM/vdebug on Linux and didn't see any code in the source window. Despite my path mapping FilePath didn't get the remaining path separators right.

There is one patch for FilePath which should fix path separators in Unix<->Windows mixed scenarios that comes with some additional unit tests. The other one fixes the pathes in the Stack Window where s.get('filename') doesn't give the local but the RemoteFilePath.

With these small fixes I'm now happily debugging in cross OS scenarios thanks to your great VIM plugin.

Kind regards,
Bernd