Support alignMode with particles

Particle (or SpriteParticle) could have "alignMode" property with enum values:

• AlighTowardsZ: (Default) This makes particles textures base direction towards Z, where camera usually is. Use Emitter particleRotation* to change direction.
• AlighTowardsCamera: This makes particles textures base direction towards camera always, even when camera moves & rotates.
• AlignTowardsStartVelocity: This makes particles textures base direction towards startVelocity. (Not changing to current direction, unless we implement a way to store previous position in our stateless system). Useful for e.g. line / lightning type of longer particles rotated properly.

Emitter particleRotation* properties are always added to these base directions. So if user want's particles to be in +/- 20-degree x angle towards camera, use AlighTowardsCamera and particleRotationVariation: Qt.vector3D(20,0,0).