flatpak: set appdata release version for dev version: appdata file can now use a template (which has the same name with "_tpl_" prefix), which is used in dev version to automatically set the release with the right Mercurial revision/date. Comment from org.salutatoi.Cagou.appdata.xml has been moved as first child of <component>, so it is not lost while parsing the file.

{
    "_build_settings": {
        "package": "sat",
        "wrapped_command": "primitivus",
        "dev_repos": {"type": "hg", "url": "https://repos.goffi.org/sat"}
    }
}