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/mercurial/mercurialplugin.cpp

@@ -643,8 +643,7 @@ void MercurialPluginPrivate::showCommitWidget(const QList<VcsBaseClient::StatusI
         return;
     }
 
-    Core::IEditor *editor = Core::EditorManager::openEditor(saver.filePath().toString(),
-                                                            Constants::COMMIT_ID);
+    Core::IEditor *editor = Core::EditorManager::openEditor(saver.filePath(), Constants::COMMIT_ID);
     if (!editor) {
         VcsOutputWindow::appendError(tr("Unable to create an editor for the commit."));
         return;