Add Appainter

[zeshuaro]

Please read How to contribute before creating a submission.

Description

Appainter is a material theme editor and generator for Flutter to configure and preview the overall visual theme of a material app.

Repo: https://github.com/zeshuaro/appainter

App link: https://appainter.dev/

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Checklist

• I read How to contribute
• I edited the SOURCE.md file only
• Added a link to the repo in the PR

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