Set StartupWMClass in .desktop file again

The Ubuntu / GNOME dock otherwise does not properly detect the correct .desktop entry. The name is the same that is used for the QApplication application name (PICARD_APP_NAME).

Author: phw

org.musicbrainz.Picard.desktop

@@ -5,6 +5,7 @@ Exec=picard %F
 Terminal=false
 Type=Application
 StartupNotify=true
+StartupWMClass=Picard
 Icon=org.musicbrainz.Picard
 Categories=AudioVideo;Audio;AudioVideoEditing;
 MimeType=application/ogg;application/x-flac;audio/aac;audio/ac3;audio/aiff;audio/ape;audio/dsf;audio/flac;audio/midi;audio/mp4;audio/mpeg;audio/mpeg4;audio/mpg;audio/ogg;audio/vorbis;audio/x-aac;audio/x-aiff;audio/x-ape;audio/x-flac;audio/x-flac+ogg;audio/x-m4a;audio/x-midi;audio/x-mp3;audio/x-mpc;audio/x-mpeg;audio/x-ms-wma;audio/x-ms-wmv;audio/x-musepack;audio/x-oggflac;audio/x-speex;audio/x-speex+ogg;audio/x-tak;audio/x-tta;audio/x-vorbis;audio/x-vorbis+ogg;audio/x-wav;audio/x-wavpack;audio/x-wma;video/x-ms-asf;video/x-theora;video/x-wmv;