Package for Flatpak

[jmaris]

Is your feature request related to a problem? Please describe.
The only real issue is that installation is quite complex, and that for the growing proportion of users not using a traditional package manager, installation can be impossible.

Describe the solution you'd like
It might make sense to ship GIMP-ML as a flatpak, as is the case for many other GIMP extensions.

Describe alternatives you've considered
As flatpak's runtimes can't be easily modified, there is no other easy way to install GIMP-ML in flatpak instances of GIMP. Providing GIMP-ML as a flatpak could allow us to add the required python libraries by extending the GIMP flatpak, as other extensions do.