Flatpak packaging for Goodvibes

Add the Flathub repository and install the necessary SDK:

$ make setup

To build the package, run:

$ make

To force rebuild (when the manifest didn't change, but we know that the remote Git repository changed):

$ make -B

This builds both the Flatpak repo (in repo) and the bundle as <app-id>.flatpak. To install the bundle:

$ make install

Then run the application:

$ make run

If things go wrong:

$ make debug
[📦] $ alias ls='ls --color'
[📦] $ alias ll='ls -l'
[📦] $ cat /etc/os-release
[📦] $ ...

When done, don't forget to cleanup:

$ make uninstall