Prepare merge tool for nameservers using add. field

[westram]

Merge tool shall resist to merge databases which have different entries in the additional field used for the nameserver.

Merging databases with different fields leads to data loss, because different species are treated as equal, i.e. sequences and fields will be overwritten with sequences from wrong species.

• Force 'generate new names' for both databases
  • when done → set flag
• Allow to set flag manually (if is really sure names are ok)
• Deny to merge species, configs or trees

[westram]

• User is forced to generate new names for both databases (which is possible with one click now - instead of two)
• Changed layout of mergetool-startup-window (vertical instead of horizontal)
• Made additional fields for nameserver editable in rename-window
• If rename generates duplicates, merging is not possible. Added a switch to override that behavior.
• Added 'auto-rename' option to overwrite-dialog (import via simple merge)