minimize_wdba: use a product to decide accepting SCCs
Fixes #347. * spot/twaalgos/minimize.cc, spot/twaalgos/minimize.hh: An SCC of the determinized automaton is marked as accepting if it intersects any accepting SCC of the original automaton. This can be checked by making a product. As a consequence the output of minimize_wdba() now always includes the original language, and minimize_obligation() only needs a single containment check. * NEWS: Mention this.