MWReferenceSearchWidget buildIndex optimisations

* Each item builds an MWReferenceModel which creates a document
  slice clone. This is very expensive and we only use the model
  for getting basic attributes, so defer the evaluation of the
  document.
* $.show is expensive and, in this case, unnecessary.

Change-Id: I99abc4c1b17f05559a9cae68b15121a8be6d23fb
Merge "MWReferenceSearchWidget buildIndex optimisations"