Add initial Flatpak packaging
This adds the files needed to distribute Purr Data as a Flatpak.
With this commit, users can build and install a Flatpak package of PD on their local machine.
I have submitted a request to add Purr Data to the Flathub app store. Flathub prefer that all necessary files apart from the packaging manifest are stored in the upstream Git repository of the application being packaged. I hope to achieve that with this MR.
Related with #564