Add screenshots to F-Droid

Screenshots will help users evaluate the appearance and functionality of [the Camera Color Picker](https://f-droid.org/en/packages/fr.tvbarthel.apps.cameracolorpicker/) before installing it.

https://f-droid.org/en/docs/All_About_Descriptions_Graphics_and_Screenshots/
https://gitlab.com/snippets/1895688
