This is the continuation of #441.

I've been investigating with the help of @redtide how to create a Flatpak release and this is the result:

To publish on Flathub we have to follow the instructions here https://github.com/flathub/flathub/wiki/App-Submission
We must also follow the App Requirements https://github.com/flathub/flathub/wiki/App-Requirements

With the changes from #459 we should be respecting the requirements.

Build instructions:
Clone this gist: https://gist.github.com/209615e2f95e00761c9d58c186d5ccea.git
Install the runtime:
flatpak install flathub org.freedesktop.Platform//21.08 org.freedesktop.Sdk//21.08
Build:
flatpak-builder --force-clean --user --install build-dir org.lite_xl.lite_xl.yml
Run:
flatpak run org.lite_xl.lite_xl

Note: don't let lite-xl open the directory after the build, it might hang because of some symlinks to /run created by flatpak-builder. I'll open an issue about this later.

Problems found:

• The scale is different from the normal version. It looks like SDL_GetDisplayDPI returns different values. With the update to the 21.08 freedesktop SDK and runtime, which brings SDL 2.0.16, this doesn't happen anymore.
• The USERDIR (set via XDG_CONFIG_HOME) is located at ~/.var/app/org.lite_xl.lite_xl/config.

To solve the second problem we have two alternatives:

• Keep the Flatpak version separated from the host version and tell users where to find the config location
• Check if the FLATPAK_ID env variable is set, then ignore what XDG_CONFIG_HOME and instead use HOST_XDG_CONFIG_HOME or HOME/.config

According to the Requirements, we should follow the first alternative:
Do not attempt to retain configs from non-flatpak installations. While possible it is recommended that flatpak installs stay self-contained.

We should also decide what other permissions to give the Flatpak.

EDIT: Updated build manifest to use the freedesktop 21.08 SDK and runtime. This fixes the scale problem.