particle-engine: Update particle velocity and position in discrete steps

This replaces the previous implementation of:

  s = ut + 1/2at^2

with:

  v = u + at

The idea is that instead of recording a particle's initial position and
deriving it's position based on that, each new update of the particle
derives it's new position based on the last position, and velocity is
compounded.
demos: Fix speed and acceleration values

Since the update to the physics simulation in ce1beb8, each force
relating to speed and acceleration has become ~20x more powerful. This
adjusts the values accordingly to bring back some sanity to the
demonstrations.
particle-engine: Fix max_age description
catherine-wheel: Minor demo property tweaks