Tuplet-related cleanups

• Remove dead code,
• Qualify std::vector,
• Eliminate Tuplet_bracket::get_common_x,
• Type-check callback arguments,
• Move Tuplet_bracket::parallel_beam to object callbacks,
• Simplify and unify connect-to-neighbor,
• Use grob::inherit-parent-property for TupletNumber.cross-staff,
• Use simple callback for Beam.knee instead of manual caching.