Added Flutter/Dart lints to project.

[gslender]

Fixed all warnings, except example which won't patch correctly if changed. All changes made were only because they are marked as bad style or potentially allows risky code to exist which should be changed to avoid future errors.

[michaelspiss]

Thanks! I fixed the remaining linter infos in the example and adjusted the duotone patch accordingly. Please have a look if everything is as you'd expect. I'd then merge.

[gslender]

Looks good to me 👍

[gslender]

Sorry... clicked the wrong button

[michaelspiss]

Thanks again!