Particles module refactoring en masse

All the little changes in one commit.

StochasticDirection -> Direction
AngledDirection -> AngleDirection
TargetedDirection -> TargetDirection
QSGCustomParticle is now Affector's implementation (C++ only refactor)
FollowEmitter -> TrailEmitter
Kill -> Age and gained lifeLeft property
ModelParticle is removed, use ItemParticle instead.
Added RectangleShape to replace Shape for the case of Rectangles
Abstract Types are no longer createable, despite sensible defaults.
GLSL variables in CustomParticle now begin with qt_
JS variables are now such that x,vx,ax are current (old ones are now
initialX, initialVX, initialAX). Same for y. A few more were extended.
Emitter::emitCap -> Emitter::maximumEmitted. Set to -1 for noCap: false
Emitter::noCap is gone.
Emitter::emitting -> Emitter::enabled
Affector::active -> Affector::enabled
Affector::collisionParticles -> Affector::whenCollidingWith
Affector::signal is gone.
PointAttractor -> Attractor
Attractor/Wander::physics -> ::aff...