Fix 524 pitched trill losing accidental.

Fix 524 pitched trill losing accidental.

- check localKeySignature more thoroughly when deciding whether
to print an accidental.  This includes checks for alteration and
measure number.
- print accidental if forced.
- move get_bar_number () from accidental-engraver.cc to context.cc
so that pitched-trill-engraver.cc can also use it.
- check secondary note events for `forced-accidental' property and pass to
engraver via trill-span-event if set.
- prevent accidental-engraver.cc setting `forced' property on non-existent
accidentals.
- add regression tests for forced-accidental and consecutive pitched trills.