Description of problem:
There seem to be some issue with the Zim AppData file and/or desktop/icon file. Zim cannot be found and/or installed using the GNOME Software application or another package manager UI based on AppStream.
Steps to Reproduce:
1. Open GNOME Software.
2. Type "zim".
This issue is happening on both Fedora 29 and Fedora 30. Probably also on Rawhide, but I am not able to test it there at the moment. There is no such issue with the Flatpak version of Zim from Flathub.