Generate screenshots for iOS

[rolandgeider]

After merging https://github.com/wger-project/flutter/pull/760 we should also generate the iOS screenshots. We have a script in integration_test that automatically does this for android (at least almost automatically, there's a couple of manual steps involved). We should adapt this so that it also generates the localized screenshots for iOS and copies them to the correct folder.

A github actions that is manually started and does everything would be the crown on top, but is not necessary

[afzal09]

can i work on this?

[rolandgeider]

hi @afzal09, sure it's yours