Fixture prop

Currently lookup tables lets you create properties on individual columns of a Table. This implementation allows to add properties (XML attributes) on the entire row of a table (attribute on a higher level XML element).

Implemented as specified in the [spec](https://docs.google.com/a/dimagi.com/document/d/1yVSotTa33_UF9dd5iuRYc80FbkqP6sIWGiiPaAP_gVQ/edit#heading=h.v1gf9mhs7nhi).
hard to tell whether this is handled elsewhere, but any need to worry about a `KeyError` here for newly migrated items?