Particle Attractor: Add method to clear m_ParticleSystems. Add attraction movement 'InSmooth/OutSmooth'

It will be good to be able to clear m_ParticleSystems for i reuse/pool prefabs that have particle systems registering to UIAttractor.
Currently smooth movement smoothes both In and Out (i.e. both on start/acceleration and end/deceleration).
It would be great to have only InSmooth or OutSmooth. Some of effects like coin collection look much more great without deceleration on the end.