Add appdata screenshots

[Felix Krull]

Waiting on issue #503 for now to use the website screenshots as reference.

Appdata screenshot recommendations:

• https://people.freedesktop.org/~hughsient/appdata/
• https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html#sect-Quickstart-DesktopApps

[Felix Krull]

Add AppData screenshot. Fixes issue #642.

→ <<cset 1d25f881a340>>

[Felix Krull]

To elaborate: I put the screenshots into "_static_standalone" so they get published and hosted with the new website. That means the screenshots URLs aren't actually valid until we switch over to the new site.