Core: Native dir separators for DMgr::getSaveFileName warning

User visible paths need native ones.

Change-Id: I4fbadd9cf63fbb9c513b45eb6001107dbc162f86
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/documentmanager.cpp

@@ -708,7 +708,7 @@ QString DocumentManager::getSaveFileName(const QString &title, const QString &pa
             if (QFile::exists(fileName)) {
                 if (QMessageBox::warning(ICore::dialogParent(), tr("Overwrite?"),
                     tr("An item named \"%1\" already exists at this location. "
-                       "Do you want to overwrite it?").arg(fileName),
+                       "Do you want to overwrite it?").arg(QDir::toNativeSeparators(fileName)),
                     QMessageBox::Yes | QMessageBox::No) == QMessageBox::No) {
                     repeat = true;
                 }