appdata: Add developer ID

[yakushabb]

Flathub requires a developer tag and developer ID. Also Appstream decided to use rdns structure for developer ID.

It allows reverse-dns IDs like sh.cozy, de.geigi or Fediverse handle (like @user@example.org)

A developer tag with a name child tag must be present. Only one developer tag is allowed and the name tag also must be present only once in untranslated form.

<developer id="tld.vendor">
  <name>Developer name</name>
</developer>

Source: https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/#name-summary-and-developer-name

The element should have a id property, containing a unique ID to identify the component developer / development team. It is recommended to use a reverse-DNS name, like org.gnome or io.github.ximion, or a Fediverse handle (like @user@example.org) as ID to achieve a higher chance of uniqueness.

Source: https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-developer