Measure rest was drawn on fix y coordinate. Now the y coordinate is calculated
taking the staff height into account so it is always vertically centered.
Also the size of the symbol is sized depending on the staff line distance.
- 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
