This perform pre- and post-processings in addition to
the LTL-to-TGBA translation.

* src/tgbaalgos/translate.cc, src/tgbaalgos/translate.hh: New files.
* src/tgbaalgos/Makefile.am: Add them.
* src/tgbaalgos/postproc.hh: Make the private part protected, so
that we can inherit from that in the translator class.
* src/bin/ltl2tgba.cc: Use the translator class to hide LTL
simplification, translation, and postprocessings.
* NEWS: Mention it.