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/appdata/appdata.gresource.xml.in

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<gresources>
+  <gresource prefix="/org/gnome/calendar">
+    <file alias="appdata">@appid@.metainfo.xml</file>
+  </gresource>
+</gresources>