Add .desktop launcher for file-browser/menu launching

packaging/av-sync-marker.desktop wraps run.sh so the app can be
launched from Cinnamon's menu or double-clicked/opened-with from a
file manager (Nemo etc.), including passing a selected video file as
the initial file to load via %f -- main.py already supported a CLI
file argument, this just gives the file manager a way to invoke it.

Installed to ~/.local/share/applications/ (user-level, no sudo) and
verified with `gio launch`, which is what a file manager's 'Open
With' actually invokes under the hood, not just desktop-file-validate.

packaging/av-sync-marker.desktop

@@ -0,0 +1,10 @@
+[Desktop Entry]
+Type=Application
+Name=A/V Sync Marker
+Comment=Fix lossless audio/video sync by marking a shared event on each track
+Exec=/home/niels/Projects/WeAreSyncing/run.sh /home/niels/Projects/WeAreSyncing/main.py %f
+Icon=video-x-generic
+Terminal=false
+Categories=AudioVideo;Video;
+MimeType=video/mp4;video/quicktime;video/x-matroska;video/x-msvideo;video/webm;video/mpeg;
+StartupNotify=true