Build System Migration & Making the Processing Flatpak part of the Official Releases

Hi everyone,

Thanks for your work on the Flathub release of Processing4!

I'm one of the maintainers of Processing, and I wanted to give you a heads up that we're in the process of migrating our build system from Ant to Gradle (see this PR: [processing/processing4#888](https://github.com/processing/processing4/issues/888)). This is important, as the structure of the distributable will change. We'd be happy to work with you to make the transition as easy as possible.

On a related note, we'd love to make the Flatpak version an official part of future releases. I’d like to hear your thoughts on how we can make this happen.

Thanks again for your work!

cc @stefterv @Zishan-Rahman