flatpak: clearer description, follow guidelines

[secretmango]

Hey there!

I am sure the app is really great. But I didnt fully get what it is useful for.

The description is pretty much the example of how descriptions should not be. Technical but not focused on "what you can do with the app".

I would remove "GTK4" and instead tell what the app is useful for.

https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/quality-guidelines/

Cheers!