QPollingFileSystemWatcherEngine: change PollingInterval to chrono::seconds

More expressive and type-safe.

Move it to the .cpp file (the only user) and make it static to avoid
creating a non-namespaced symbol.

Pick-to: 6.6
Change-Id: Ib9b393e660e51c1791ed3a9a4a12e5c75736bf2a
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/io/qfilesystemwatcher_polling.cpp

@@ -2,11 +2,18 @@
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
 
 #include "qfilesystemwatcher_polling_p.h"
+
 #include <QtCore/qscopeguard.h>
 #include <QtCore/qtimer.h>
 
+#include <chrono>
+
+using namespace std::chrono_literals;
+
 QT_BEGIN_NAMESPACE
 
+static constexpr auto PollingInterval = 1s;
+
 QPollingFileSystemWatcherEngine::QPollingFileSystemWatcherEngine(QObject *parent)
     : QFileSystemWatcherEngine(parent)
 {

src/corelib/io/qfilesystemwatcher_polling_p.h

@@ -27,8 +27,6 @@
 QT_REQUIRE_CONFIG(filesystemwatcher);
 QT_BEGIN_NAMESPACE
 
-enum { PollingInterval = 1000 };
-
 class QPollingFileSystemWatcherEngine : public QFileSystemWatcherEngine
 {
     Q_OBJECT