Screenshots on demand

**Is your feature request related to a problem? Please describe.**
- [Many people want screenshots for some apps but not others](https://github.com/Droid-ify/client/issues/119#issuecomment-1278477228).
- For some minimalistic people screenshots are unnecessary distractions.

**Describe the solution you'd like**
On demand manually fetch screenshots of user preferred apps, which is useful for low-network situations.

**Describe alternatives you've considered**
[F-Droid Classic](https://www.f-droid.org/packages/eu.bubu1.fdroidclassic/) has this feature.

**Additional context**
Screenshot 👇