Mark KTorrent as a single main window program

As it happens with other programs (https://github.com/nextcloud/desktop/pull/4194 , pim/knotes@70e81d72 , https://github.com/qbittorrent/qBittorrent/pull/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.