Clarify the icon_root paramter in as_store_from_file()

We don't want to build the full path where the origin is specified in the XML.
trivial: Actually accept deprecated tags again
Add as_store_from_xml()

This allows us to load a store from some XML markup.
Add an 'api-version' property to AsStore to generate old-style metadata

This allows us to keep up with the latest API and still write metadata
compatible with old applications not using libappstream-glib or libappstream.
Add the new AsUrlKind's in API 0.6