Mark KDE Partition Manager as a single main window program

As it happens with other programs (nextcloud/desktop#4194 ,
https://invent.kde.org/pim/knotes/-/commit/70e81d7286c83ff28deadb2f2b80e67b463d1db7 ,
qbittorrent/qBittorrent#15996 ,
https://nicolasfella.de/posts/taskbar-single-app ,
https://gitlab.freedesktop.org/xdg/xdg-specs/-/merge_requests/53):

There can only be one instance of the program running. Mark it as such in
the .desktop file. This allows desktop environments to adjust their UI
accordingly.

src/org.kde.partitionmanager.desktop

@@ -167,6 +167,7 @@ Comment[zh_TW]=管理磁碟、分割區與檔案系統
 Exec=partitionmanager
 Icon=partitionmanager
 Categories=System;Filesystem;KDE;Qt;
+SingleMainWindow=True
 X-KDE-StartupNotify=true
 Keywords=partition;disk;
 Keywords[ar]=أقسام;قرص;