Make a flatpak and put it on flathub

[jmfergeau]

I got a report of someone who got surprised to have a junked outdated french translation on the game when they got it from flatpak. After inspection, it appears it's not the NX-evo version there but the regular outdated nx from this repository.

It would be relevant to propose cave story NX-evo directly as a flatpak in flathub. I Don't know how to do this, though. Also maybe ask the removal of regular nx on flathub if it's ever possible and unless there's a real good reason to keep regular nx there.

[jmfergeau]

So i've made some tries in my end and it seems all is already made for making the game as a flatpak. Oops.

Though, it requires the freedesktop platform at version 20.08 which is obsolete for some time. I'm going to see if updating it is at my level and eventually send a PR if yes.

Only remains posting it on flathub.

EDIT: I could compile a flatpak using freedesktop.platform 23.08. Installed and tried for 5 minutes, no problems at sight. I'm gonna make a PR for this.