Windows path in EditorInputUtil

While debugging path issues in Rascal MPL, I found two issues here:
* wsRoot.getLocation().toOSString() will on windows look like c:\folder\file.ext, while path is /c:/folder/file.ext, so using toString instead of toOSString will make a better match here
* on windows there is a leading slash missing, as required by the java URI implementation (other systems are more tolerant for this)