Pick a placeholder icon set and add it to the repo

[alice-i-cecile]

trafficstars

Should be under assets/icons, with a license attached. Related to #44.

Possible icon sets: https://iconify.design/

We're looking for something sketchy / draft, that's quick to roughly match with minimal skill. Good coverage for 3D / editor-style verbs is a bonus. These are not final icons.