Extend attractor API

Conside adding couple properties into attractor:

• "bool useCachedPositions" which would default to true (current behavior). When set to false, m_shapePositionList is not generated and position is taken live from the shape in affectParticle(). This way attractor works also when shape rotation, scale etc. changes during the particle system run.
• "int positionsAmount" which would set the amount of cached positions, so only affects when cachedPositions is true. Currently amount is same as affected particles amount, but users may want to use bigger amount to make it more random or smaller amount to save memory.