Add double-clickable launcher inside the project folder itself

Same Exec as packaging/av-sync-marker.desktop, but placed at the repo
root so it shows up right in the file manager when browsing to
WeAreSyncing/, not just in the app menu. Nemo requires such an
in-folder .desktop file to be both executable and marked 'trusted'
(gio set ... metadata::trusted true) before it'll launch without a
security prompt -- done locally, but note that trust flag lives in
GVFS's metadata store keyed off this file's inode, not in the file
itself, so it won't survive the file being rewritten (e.g. by a fresh
git checkout) and may need to be re-set with the same gio command.

A-V 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