Refactored Code to be more efficient/elegant

Improved adding/deletion of particles
Improved step-wise updating of particles
Removed global constants for velocity and gravity adjustments
Made tracker more efficient by changing from tracking Particles to
Particle* (probably should have done this a while ago)
Changed tracker to true singleton
-Bug left: particles are made together in top-left corner -> look at
input code
-Plan: update SFML to latest