Add icons for launcher actions

### Problem

Launcher actions should have icons so they can be easily identified in search, and in the future keyboard shortcuts

### Proposal

We have custom icons for the selection modes already, but they need to be resized and exported under the screenshot namespace:
- [ ] Area
- [ ] Window
- [ ] Screen

### Prior Art (Optional)

exporting an icon under the app's namespace: https://github.com/elementary/calendar/pull/837