Add screenshots to F-Droid

[Korb]

Screenshots will help users evaluate the appearance and functionality of the xray-plugin before installing it.

https://f-droid.org/en/docs/All_About_Descriptions_Graphics_and_Screenshots/
https://gitlab.com/snippets/1895688

[IzzySoft]

Would be solved via #10 – but unfortunately, no response from @teddysun