The thing is that the application launcher always shows the system default application icon instead of the icon I set in the .desktop file. I use the same install locations and files for Debian packaging successfully so I guess they are correct.
Source code is here if someone gets interested:
Running ./scripts/buildSnap.sh builds the snap package.
Also, I would like to point out that not all applications in a single snap will prefer a single icon, that’s probably why the Icon key in the desktop entries are not patched in the first place, because Snapcraft doesn’t know what the proper icon belongs to a certain menu entry. This probably should be handled by the packager themselves.
NOTE: The actual desktop entry effective are in /var/lib/snapd/desktop and is generated by snapcraft according to the desktop entry specified in the desktop key of an apps instance.
I don’t think so, the apps.<app-name>.desktop key only allows relative paths from the prime folder. AFAICS the /snap/gui folder will be copied to /prime/meta/gui by snapcraft, so setting apps.<app-name>.desktop key to meta/gui/heimer.desktopmight work (never done that before :-/)