Fixed that File > Open With was not working with already open file

It was just activating the existing editor, instead of reopening with
the new editor type.

Change-Id: I1fee08d1df1d78a32298d222a09cf05cf9433f19
Task-number: QTCREATORBUG-14336
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>

src/plugins/coreplugin/mainwindow.cpp

@@ -843,7 +843,7 @@ void MainWindow::openFileWith()
         if (isExternal)
             EditorManager::openExternalEditor(fileName, editorId);
         else
-            EditorManager::openEditor(fileName, editorId);
+            EditorManagerPrivate::openEditorWith(fileName, editorId);
     }
 }