Update to Eigen3 API.
Monitor FPS.
Exit when Q is pressed.
Move flockers towards mouse presses, predators flee.
Cycle brush patterns on Predators.
Performance optimizations.

+ Use QtConcurrent to parallelize the direction updates and stepping
  processes.
+ Remove qobject_casts by introducing Entity::EntityType.
Increase default FPS to ~83.3.
Make predators more aggressive. And sneaky.

The predators now have more stealthy blink behaviors, and are thus more
capable of capturing their prey.
Parameter/behavior tweaks for flocking behaviors.