MRACPriorInputEditor not picking up the right taxon sets

[rbouckaert]

When specifying taxonsets via the snapper template, the taxa in the MRCAPriorInputEditor

1. are displaying too much information in the list box, since they include all sub-taxa,
2. are not updated when they are edited in the TaxonSetInputEditor leading to incorrect XML
   See using a MRCA + snapper made mismatches with Taxon ~ TaxonSet labels beast2#1106 for an example.

[rbouckaert]

When renaming a taxon, then do a "fill down" in the taxon editor it works fine now. However, adding an MRCAPrior using the newly named taxon leads to creation of invalid XML: a new Taxon is created instead of a reference to the existing taxon set, and the ID has a number added to it (maybe related to CompEvol/beast2#237).