Custom application icon do not appear in Gnome and LXQT

[Yvan Masson <yvan@masson-informatique.fr>]

Hi,

I made a small graphical app with Python/GTK. When installing it 
(manually or via pip3), it properly appears in the software list.

However, after starting the app, Gnome does not show my icon but a 
default one, indicating it is missing or invalid. I installed LXQT to 
compare, and result is identical (no icon at all after application 
started). Tests were done on my laptop running Debian testing.

I ran `gtk-update-icon-cache` in the proper directory to create an icon 
theme cache, but it does not help.

This issue is driving me crazy :-)

Code is on https://framagit.org/Yvan-Masson/LibreTrombi
Icons are in directory `LibreTrombi/data/icons/hicolor`

Any idea would be greatly welcome!

Regards,
Yvan