* src/tgba/sba.hh: New file, extrated from...
* src/tgba/tgbaexplicit.hh: ... here.  Also rename
sba_explicit::is_accepting as sba_explicit::state_is_accepting for
consistency with tgba_sba_proxy.
* src/tgbatest/explicit2.cc: Adjust to the renaming.
* src/tgba/Makefile.am: Add sba.hh.