about, appdata: Port to `AdwAboutDialog.new_from_appdata`

This removes a lot of duplicated code, as we can simply reuse the appdata file. It will also allow us to add release notes without needing to rewrite them. A simple update in the adw_about_dialog_new_from_appdata method will take care of displaying the release notes.

This also renames the appdata file from appdata.xml to metainfo.xml, as appdata.xml is considered a legacy filename.