desktop entry: fix duplication of the app icon in a dock like Plank

Without this entry, when starting mpv as a flatpak application on
elementaryOS, the Dock icon would get duplicated, as if the application
was detached from its launcher. This entry fixes that by allowing the
dock to associate the app's window with the desktop entry.

Fixes mpv-player#9109

etc/mpv.desktop

@@ -34,3 +34,4 @@ Terminal=false
 Categories=AudioVideo;Audio;Video;Player;TV;
 MimeType=application/ogg;application/x-ogg;application/mxf;application/sdp;application/smil;application/x-smil;application/streamingmedia;application/x-streamingmedia;application/vnd.rn-realmedia;application/vnd.rn-realmedia-vbr;audio/aac;audio/x-aac;audio/vnd.dolby.heaac.1;audio/vnd.dolby.heaac.2;audio/aiff;audio/x-aiff;audio/m4a;audio/x-m4a;application/x-extension-m4a;audio/mp1;audio/x-mp1;audio/mp2;audio/x-mp2;audio/mp3;audio/x-mp3;audio/mpeg;audio/mpeg2;audio/mpeg3;audio/mpegurl;audio/x-mpegurl;audio/mpg;audio/x-mpg;audio/rn-mpeg;audio/musepack;audio/x-musepack;audio/ogg;audio/scpls;audio/x-scpls;audio/vnd.rn-realaudio;audio/wav;audio/x-pn-wav;audio/x-pn-windows-pcm;audio/x-realaudio;audio/x-pn-realaudio;audio/x-ms-wma;audio/x-pls;audio/x-wav;video/mpeg;video/x-mpeg2;video/x-mpeg3;video/mp4v-es;video/x-m4v;video/mp4;application/x-extension-mp4;video/divx;video/vnd.divx;video/msvideo;video/x-msvideo;video/ogg;video/quicktime;video/vnd.rn-realvideo;video/x-ms-afs;video/x-ms-asf;audio/x-ms-asf;application/vnd.ms-asf;video/x-ms-wmv;video/x-ms-wmx;video/x-ms-wvxvideo;video/x-avi;video/avi;video/x-flic;video/fli;video/x-flc;video/flv;video/x-flv;video/x-theora;video/x-theora+ogg;video/x-matroska;video/mkv;audio/x-matroska;application/x-matroska;video/webm;audio/webm;audio/vorbis;audio/x-vorbis;audio/x-vorbis+ogg;video/x-ogm;video/x-ogm+ogg;application/x-ogm;application/x-ogm-audio;application/x-ogm-video;application/x-shorten;audio/x-shorten;audio/x-ape;audio/x-wavpack;audio/x-tta;audio/AMR;audio/ac3;audio/eac3;audio/amr-wb;video/mp2t;audio/flac;audio/mp4;application/x-mpegurl;video/vnd.mpegurl;application/vnd.apple.mpegurl;audio/x-pn-au;video/3gp;video/3gpp;video/3gpp2;audio/3gpp;audio/3gpp2;video/dv;audio/dv;audio/opus;audio/vnd.dts;audio/vnd.dts.hd;audio/x-adpcm;application/x-cue;audio/m3u;
 X-KDE-Protocols=ftp,http,https,mms,rtmp,rtsp,sftp,smb,srt
+StartupWMClass=mpv