Add FormulaManagerView.close() that closes the FormulaManager
such that client code does not need to keep a reference
to the underlying FormulaManager (which it should not use anyway).

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14908 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c