Detect system dark mode preference changes without restart on Linux

The new org.freedesktop.appearance.color-scheme key allows us to do this
in a reliable way.

Recently freedesktop standardized the system dark mode preference in a
desktop environment independent way in the xdg-desktop-portal
specification.

The specification can be seen here: d7a304a006/data/org.freedesktop.impl.portal.Settings.xml (L33-L45)

KDE supports this since KDE Plasma 5.24 and Gnome supports this since
Gnome 42.

Relevant blog post: https://blogs.gnome.org/alexm/2021/10/04/dark-style-preference/

Fixes #7146

src/gui/osutils/nixutils/NixUtils.h

@@ -21,6 +21,7 @@
 #include "gui/osutils/OSUtilsBase.h"
 #include <QAbstractNativeEventFilter>
 #include <QSharedPointer>
+#include <QtDBus/QDBusVariant>
 
 class NixUtils : public OSUtilsBase, QAbstractNativeEventFilter
 {
@@ -48,6 +49,9 @@ public:
         return false;
     }
 
+private slots:
+    void handleColorSchemeChanged(QString ns, QString key, QDBusVariant value);
+
 private:
     explicit NixUtils(QObject* parent = nullptr);
     ~NixUtils() override;
@@ -66,6 +70,16 @@ private:
     };
     QHash<QString, QSharedPointer<globalShortcut>> m_globalShortcuts;
 
+    // defined as per "org.freedesktop.appearance color-scheme" spec in
+    // https://github.com/flatpak/xdg-desktop-portal/blob/d7a304a00697d7d608821253cd013f3b97ac0fb6/data/org.freedesktop.impl.portal.Settings.xml#L33-L45
+    enum ColorschemePref
+    {
+        PreferNone,
+        PreferDark,
+        PreferLight
+    };
+    ColorschemePref m_systemColorschemePref = ColorschemePref::PreferNone;
+
     Q_DISABLE_COPY(NixUtils)
 };