Mark Amarok 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.amarok.desktop

@@ -191,6 +191,7 @@ Comment[x-test]=xxAmarok - Rediscover Your Music!xx
 Comment[zh_CN]=Amarok - 重新感悟音乐之美！
 Comment[zh_TW]=Amarok - 重新探索您的音樂世界！
 Icon=amarok
+SingleMainWindow=True
 # Add pure audio formats here, for other formats see amarok_*.desktop files
 # See bug 242292
 MimeType=audio/aac;audio/mp4;audio/mpeg;audio/mpegurl;audio/vnd.rn-realaudio;audio/vorbis;audio/x-flac;audio/x-mp3;audio/x-mpegurl;audio/x-ms-wma;audio/x-musepack;audio/x-oggflac;audio/x-pn-realaudio;audio/x-scpls;audio/x-speex;audio/x-vorbis;audio/x-wav;application/x-ogm-audio;audio/x-vorbis+ogg;audio/ogg;