graph: store the source indices in the transition vector
... and use it to sort transitions. * src/graph/graph.hh: Adjust storage of source index. Provide remove_dead_transitions_(), sort_transitions_() and chain_transitions_() methods. * src/tgba/tgbagraph.cc (merge_transitions): Rewrite using above methods. * src/tgba/tgbagraph.hh: Add a comparison operator for transitions. * src/tgbatest/degenlskip.test, src/tgbatest/det.test, src/tgbatest/ltl2ta.test, src/tgbatest/neverclaimread.test, src/tgbatest/readsave.test: Adjust expected transition order in test cases.