add flatpak support

This simplifies testing gajim on distributions that haven't the latest/required dependencies (like the avahi python3 bindings).

If you haven't installed flatpak check http://flatpak.org/getting.html for a how-to.

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Run the following steps from a directory containing the gajim source dir.

Install gajim flatpak repo

1. flatpak --user remote-add --from gnome https://sdk.gnome.org/gnome.flatpakrepo
2. flatpak --user install gnome org.gnome.Platform//3.24
3. flatpak --user install gnome org.gnome.Sdk//3.24
4. flatpak-builder --repo=repo directory gajim/org.gajim.Gajim.json
5. flatpak --user remote-add --no-gpg-verify repo repo
6. flatpak --user install repo org.gajim.Gajim
7. flatpak run org.gajim.Gajim

Update gajim flatpak repo

1. update your gajim source repository
2. rm -r directory
3. flatpak-builder --repo=repo directory gajim/org.gajim.Gajim.json
4. flatpak --user update

Note: remove --user if you want a system-wide installation