Make Nheko show up in system notification settings on Linux

relates to #845

resources/nheko.desktop

@@ -9,3 +9,4 @@ Categories=Network;InstantMessaging;Qt;
 StartupWMClass=nheko
 Terminal=false
 MimeType=x-scheme-handler/matrix;
+X-GNOME-UsesNotifications=true

src/notifications/ManagerLinux.cpp

@@ -146,6 +146,8 @@ NotificationsManager::systemPostNotification(const QString &room_id,
     QVariantMap hints;
     hints["image-data"]    = icon;
     hints["sound-name"]    = "message-new-instant";
+    hints["desktop-entry"] = "nheko";
+
     QList<QVariant> argumentList;
     argumentList << "nheko";  // app_name
     argumentList << (uint)0;  // replace_id