Rename IFile->IDocument and FileManager->DocumentManager

And adapt the other API respectively.

Change-Id: I1e04e555409be09242db6890f9e013396f83aeed
Reviewed-by: Bill King <bill.king@nokia.com>
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>

src/plugins/imageviewer/imageviewer.cpp

@@ -131,7 +131,7 @@ bool ImageViewer::open(QString *errorString, const QString &fileName, const QStr
     return true;
 }
 
-Core::IFile *ImageViewer::file()
+Core::IDocument *ImageViewer::document()
 {
     return d->file;
 }