Compute+display velocities

While here: refactor TrajSampler to return a generic 'Trajectory'
that can store various data for each point.
Display sampling duration
Memoization on Bezier patch length computation

To this end: BezierPath::curves now holds pointers.
Memoization for 'paramAtDist'

Much better performances, always well below 100ms in debug mode.

While here:
- fought with C++ initializations... had to refactor
  'addifclose' to prevent modifying inplace the length.
  I'm not good enough in programming to play with these
  language that do not take care of memory for you...