Fix AlignmentHandle::ToString for long sequence names

If the sequence name is longer than width/4, we chop the rightmost
characters of the sequence name and add an ellipsis.

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@2710 5a81b35b-ba03-0410-adc8-b2c5c5119f08