refactoring/rewriting ErrorPathShrinker. Removing the use of a big, complex inner class. Now it simply iterates over the path backwards (much more similiar to a transferRelation than before). It also handles MultiEdges (this was the problem of #160 ). It uses the new 'qualifiedName'-feature instead of the ugly tracking of global variables.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13182 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c