I tried for hours to add desktop icons and symbolic icons to my app but it didn't work.
It took me 4!!! hours to notice that there is a difference between really installing an app and running it with #GNOME Builder.
Yes, afterwards it is obvious but for a newbie like me this wasn't.
So if you are new in the game: App icons only appear if you install the app so every file is at the right place on your system. Pressing play in GNOME Builder is not the same! 😄