Default font misses most unicode icons

[Athemis]

Describe the bug
The egui default font (nice as it is...) misses some unicode icons such as e.g. cross: ✕. Since we used it in
some occations, that leads to ugly placeholders shown instead.

To Reproduce
Steps to reproduce the behavior:

1. Add a keyword.
2. Click on the keyword to edit
3. Observe missing icon on abort button.

Expected behavior
Meaningful icon should be shown

Screenshots