Add `desktop-entry' hint to D-Bus notifications.

The notification system in KDE Plasma 5.16+ requires a 'desktop-entry'
hint in the D-Bus message to enable per-application notification
settings.

src/widgets/osd_x11.cpp

@@ -111,6 +111,7 @@ void OSD::ShowMessageNative(const QString& summary, const QString& message,
     hints["image_data"] = QVariant(image);
   }
 
+  hints["desktop-entry"] = QVariant(QCoreApplication::applicationName());
   hints["transient"] = QVariant(true);
 
   int id = 0;