Now that we support any type of omega-accetpance, not just "Generalized
Büchi", it is time to move away from GB and replace it by "ω", written
just w in ASCII.

This just change the name of the tgba class.  This part has to be done
by hand because the word "tgba" occurs in many contexts and a mass
replacement would be wrong.

This will be followed by some automatic renaming of all the derived
types and more.

* src/bin/autfilt.cc, src/bin/randaut.cc, src/kripke/fairkripke.hh,
src/ta/tgta.cc, src/ta/tgta.hh, src/tgba/fwd.hh, src/tgba/taatgba.cc,
src/tgba/taatgba.hh, src/tgba/tgba.cc, src/tgba/tgba.hh,
src/tgba/tgbagraph.hh, src/tgba/tgbaproduct.cc, src/tgba/tgbaproduct.hh,
src/tgba/tgbaproxy.cc, src/tgba/tgbaproxy.hh,
src/tgba/tgbasafracomplement.cc, src/tgba/tgbasafracomplement.hh,
src/tgbaalgos/are_isomorphic.cc, src/tgbaalgos/cleanacc.cc,
src/tgbaalgos/dotty.cc, src/tgbaalgos/dupexp.cc,
src/tgbaalgos/dupexp.hh, src/tgbaalgos/hoa.cc,
src/tgbaalgos/neverclaim.cc, src/tgbaalgos/simulation.cc,
src/tgbaalgos/stutter.cc, src/tgbatest/checkpsl.cc,
src/tgbatest/emptchk.cc, src/tgbatest/ltl2tgba.cc, wrap/python/spot.py,
wrap/python/spot_impl.i: Rename the tgba class into twa.