Update bodies after changing pixelsPerMeter

Only active dynamic bodies are currently updated during the following
world step.

This patch forces an update of body and fixture underlying meter
metrics according to their target's current pixel values but using
the new pixelsPerMeter value for the conversion.

It's then the responsibility of the application to make sure that
bodies and fixtures using meter coordinates (i.e. values multiplied
by World.pixelsPerMeter) are updated before the next world step
using property bindings or an onPixelsPerMeterChanged handler.

This follows fixes for issue #28.