Core: Add FilePath based overloads to EditorManager::openEditor{,at}

Part of the overall FilePath migration. Keep the original version for
a while to allow using code to catch up.

Change-Id: Ia7c5ea14416a06e679e8661c0e4045981db87b9b
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/mainwindow.cpp

@@ -950,7 +950,7 @@ void MainWindow::openFileWith()
         if (isExternal)
             EditorManager::openExternalEditor(fileName, editorId);
         else
-            EditorManagerPrivate::openEditorWith(fileName, editorId);
+            EditorManagerPrivate::openEditorWith(FilePath::fromString(fileName), editorId);
     }
 }