- tick names a position on the time axis
- tick is always a Fraction()
- only Measure() and Segment() (and Tuplet?) have a tick value
- tick() for an generic element return only a sensible value if isMeasure() or isSegment() or isSegment(parent())
- "ticks" names a duration stored in a Fraction()
- the tick value for an Segment is relative to its measure
- rename "duration" to "ticks"
- rename afrac() to tick()
- rename rfrac() to rtick()
- rename some variables, changing "fraction" into "tick"
(example: actualFraction() into actualTicks())
- Lyrics ticks are written as Fraction, on read if xmlreader sees a "/" it reads a fraction
else midi ticks for backwards compatibility
The problem is deep in the initialization of staff types in Staff.
This commit also tries to make handling of StaffType more "const correct" and adds some
assertions which may lead to crashes (and helps detecting bad code).
