MRCAPriorInputEditor does not update taxonset for MSC

[rbouckaert]

To reproduce:

• Choose snapper template in BEAUti, and add hemi129.nex alignment.
• In the priors panel, add MRCAPrior, there now is a choice of 4 taxa.
• Go to Partitions panel and merge two taxa, say fas and kya and call it something else, say faskya.
• Go back to the priors panel, add another MRCAPrior, and instead of having 3 taxa (including faskya) there are still the previous 4 old taxa.

The MRCAPriorInputEditor does not pick up the edited taxon set. For snapper, the taxon set comes with a filtered alignment, but for *BEAST it does not, so there are different configurations for obtaining the taxon set. Perhaps it is simpler to make the taxon set editor reuse existing taxa and rename them when not in use, and propagate changes through the relevant BEAST objects.