Improve consistency of title definition in the XML

Titles are defined in the schema for some fields but not others. This means they can't be effectively leveraged for things like Views & Drupal Entity integration (and is probably the reason they are individually hard-coded into the views code in Drupal 7).

In order to put this on the path of improvement I'm going to add a test to check they are defined. Unfortunately we'll have to skip lots of entites to start off with as fixing all at once would be quite big.