Auto-generate screenshots with reasonable text

Something comparable to the screenshot currently shown on https://briarproject.org/download-briar-desktop/ or the Android screenshots on Google Play.

For this we need (in a separate build configuration):

• some nice conversations, also including special "messages" like introduction requests (can be added to our deterministic test data)
• ability to fake some contact connection statuses in code (show contacts as connected for screenshot)
• usage of compose built-in snapshot feature (see https://dev.to/pchmielowski/automate-taking-screenshots-of-android-app-with-jetpack-compose-2950 as reference)