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.
Hari Rana
0c8e1834f7