Add custom emission capability to FollowEmitter

Also implement speedFromMovement (a convenience now) and fix
a bug in QSGParticleData::curSize()

Change-Id: I831494f24f4b4afaee47524e4a4060f06fefdb0a
Reviewed-on: http://codereview.qt.nokia.com/3396


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>