Fix deprecated top-level developer_name in AppData XML

[musicinmybrain]

Use the name element in a developer block instead, as recommended by appstreamcli 1.0.0.

This fixes all warnings when validating the AppData XML file, although there are still informational messages:

I: com.github.donadigo.appeditor.desktop:31: description-first-para-too-short
     Edit application entries shown in application menu and their properties.
   The first `description/p` paragraph of this component might be too short (< 80 characters).
   Please consider starting with a longer paragraph to improve how the description looks like in
   software centers and to provide more detailed information on this component immediately in the
   first paragraph.

I: com.github.donadigo.appeditor.desktop:92: developer-id-missing
   The `developer` element is missing an `id` property, containing a unique string ID for the
   developer. Consider adding a unique ID.

I: com.github.donadigo.appeditor.desktop:~: desktop-app-launchable-omitted
   This `desktop-application` component has no `desktop-id` launchable tag, however it contains all
   the necessary information to display the application. The omission of the launchable entry means
   that this application can not be launched directly from installers or software centers. If this
   is intended, this information can be ignored, otherwise it is strongly recommended to add a
   launchable tag as well.

[musicinmybrain]

Let me clarify that appstream 1.0.0 has not yet been released, and my appstream is a pre-release as currently packaged in Fedora Linux Rawhide (the development version of the OS), corresponding to ximion/appstream@d88ed03. See also ximion/appstream#244.

[musicinmybrain]

As of appstream 1.0.2, the deprecated developer_name tag has been downgraded to info (I think this happened in 1.0.1), but in this PR, appstreamcli validate has started warning (and exiting with nonzero status) about the lack of an id property on the developer element:

W: com.github.donadigo.appeditor.desktop:92: developer-id-missing
   The `developer` element is missing an `id` property, containing a unique string ID for the
   developer. Consider adding a unique ID.

Per https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-developer, “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.”

I’m modifying this PR to propose using com.donadigo as the developer ID.