Add some more properties to JS particle type

Add some more properties to JS particle type

Adds the current position, velocity and acceleration.

While we're at it, renaming the velocity vars from sx,sy to vx,vy

Change-Id: I87f582169c7873c45a6fd453052b25c4dc940f3e
Reviewed-on: http://codereview.qt.nokia.com/2021
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>