Improve the flatpak-clean command and docs

[vemonet]

@tom-james-watson thanks for all the improvements! (cf. #89) I have tried the last version and it works well

I updated the make flatpak-clean command to also delete user data when uninstalling the app (to make sure everything is properly reset when developing), and improved a bit the flatpak debug part in the readme

Regarding the issues with the light/dark mode:

I've actually switched my system to Fedora 38 (as opposed to Ubuntu 22.04 which is what I was running before). On this system the app doesn't seem to respect dark mode - it is always rendered in light mode. Not really sure why at this point. It seems like this is something that is handled a lot better if we use libadwaita, which would be part of the gtk4 migration.

On my laptop (Fedora 38) Emote uses the dark mode, which was what I set system-wide. But if I switch the system to light mode, Emote does not pick it up and stays in dark mode...

I could not find anything about light/dark mode in the codebase though, it is supposed to be handled automatically by GTK3? So if that bugs the best would be to work on GTK4 migration?

The next step now would be to publish to flathub then?

[tom-james-watson]

Yeah the next step is to publish the flatpak. I started looking into it but I was hitting issues (#89 (comment)) and ran out of steam. So yup, next step would be to work out how to get the repo into a state that flathub is happy with.

A kind soul also worked out how to upgrade to core22 for snap (#96), so once we have flatpak working again, we can undo the change to remove snap and merge that PR in. We can focus on getting flatpak fully working first though, that's the priority IMO.