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.

src/meson.build

@@ -64,6 +64,7 @@ calendar_incs = [
 ###################
 
 sources = files()
+subdir('appdata')
 subdir('core')
 subdir('gui')
 subdir('search')