Publish Flatpak on Flathub

[ninjawarrior1337]

Flatpak allows applications to be compatible with almost all linux distros because they are compiled and run in a container-like environment. You can know more about flatpak and flathub at https://flatpak.org/ and https://flathub.org/

I have a working flatpak here: https://github.com/ninjawarrior1337/flathub/tree/cc.craftos_pc.craftos_pc

There are a couple issues with how CraftOS-PC is being deployed on linux currently which is preventing deployment there:

• Lack of appdata.xml file upstream for the app to show project info in Linux app stores such as Gnome Software and KDE Discover for which there is a suitable one in the repo above.
• Incorrectly sized icons in the resources/linux-icons.zip file (file marked 64x64 actually being 1024x1024).

I am able to submit a pull request containing these fixes.

[MCJack123]

Alright, I've added an appdata.xml into resources, and fixed the icon issue (I now publish more sizes as well).

For the record, I personally don't like Flatpak (or Snap) very much, but I don't mind anyone else publishing and maintaining a Flatpak for CraftOS-PC or any of my other code. Feel free to distribute my stuff however you want, as long as it follows the project license.