Simplify or document launching external editors

[lostfictions]

Setting an external editor in the Flatpak version of Godot is trickier than it seems like it should be.

If I go to Editor > Editor Settings... > Text Editor > External and set the following configuration:

It fails with the warning (logged on the host only):

**ERROR** OS_Unix::execute - Could not create child process while executing: code

Eventually I figured out I could make it work by using flatpak-spawn as the executable instead, and prefixing --host code as additional arguments:

But this isn't obvious at all, especially to folks who are used to Flatpak applications being set up to "just work". It would be nice if this could either be simplified or clearly documented (maybe on its Flathub page) so that other users don't get caught on the same problem.

[Calinou]

This is already documented in the README: https://github.com/flathub/org.godotengine.Godot#using-an-external-script-editor

Unfortunately, Flathub doesn't really encourage going to the GitHub repository from its description page. I could modify the AppStream metadata description to mention Flatpak-specific caveats, but it feels tedious to have to do this.
Edit: Done here: #69

Perhaps we could patch Godot in the Flatpak build to make this more seamless to the user. Exec Path would just be a variable for Exec Flags to use, and flatpak-spawn would always be run behind the scenes.