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.

Combined with #148, this fixes all warnings when validating the AppData XML file, although there are still informational messages:

I: com.github.dahenson.agenda:15: 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.dahenson.agenda:21: url-not-secure http://brainofdane.com
   Consider using a secure (HTTPS) URL for this web link.

I: com.github.dahenson.agenda:33: description-first-para-too-short
     A task manager to help you keep track of the tasks that matter most.
   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.

✔ Validation was successful: infos: 3, pedantic: 2

For url-not-secure, I plan to open a separate PR.

[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]

Now that appstream is up to 1.0.2, the deprecated developer_name tag is downgraded to info (I think this happened in 1.0.1).

I: com.github.dahenson.agenda:15: developer-name-tag-deprecated
   The toplevel `developer_name` element is deprecated. Please use the `name` element in a
   `developer` block instead.

but in this PR, it’s started warning (and exiting with nonzero status) about the lack of an id property on the developer element:

W: com.github.dahenson.agenda:15: 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 @[email protected]) as ID to achieve a higher chance of uniqueness.”

I’m rebasing this on current master and suggesting com.brainofdane as a possible developer ID.