Fix: Shouldn't be able to edit Taxonomic Group

When editing an IdentificationUnit, one should not be able to change the
Taxonomic group. This could lead to situations, where an IU has a new
Taxonomic group but still analyses from its old one which would now be
illegal.

Fixes #58
Reload Properties on Refresh Vocabulary

Fixes #13
- Display an info lable when there are no more unset Properties for the
  event