Better "portable" binaries for linux

[codeling]

The current binaries for linux are packaged on Linux Mint 18.1, and will only run on similar distributions / on linux with libstdc++ of at least 3.4.21.

We are investigating ways to produce more portable binaries suitable for a broader range of distributions. Potential solutions:

• flatpak (current state: the sandboxing so far has prevented from creating a useful binary - access to files outside of the sandbox should be possible, but was not working so far). A yml build recipe is available on request. Update: Recent experiments with flatpak builds went better. Next versions of open_iA will probably be available via flatpak!
• pkg2appimage - not explored yet, looks promising.
• docker - it doesn't seem possible to do GUI applications with it.

[codeling]

Starting with 2020.04, we provide a flatpak repository. See the 2020.04 version release notes