Use annotations on Histology work list to indicate progress

[Nicklas Nordborg]

This is related to #557 which solved the problem with deleting registered Histology work lists by changing the external id. It would be better to use annotations to keep track of this. For example, when the paraffin blocks are registered the work list is annotated with the EmbeddingDate annotation. Then we can use this annotation for filtering instead of the external id.

Requires BASE 3.3 (​http://base.thep.lu.se/ticket/1795)