Fixes the 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