Manager: Fix style not reacting to Windows light/dark mode change

Add a native event filter for dark mode update, move the function
that queries the current theme to qt_util.cpp and make widgets
with custom stylesheets update their style on update

src/qt/qt_vmmanager_main.hpp

@@ -80,6 +80,9 @@ public slots:
     void modelDataChange();
     void onPreferencesUpdated();
     void onLanguageUpdated();
+#ifdef Q_OS_WINDOWS
+    void onDarkModeUpdated();
+#endif
     void onConfigUpdated(const QString &uuid);
     int  getActiveMachineCount();