Fixed bug that particles destroyed by phyics wouldn't be taken from
dustCount
Made spawning circular by default, deprecated spawnDust and replaced
with spawnCircle as a companion to spawnRect. He gets lonely. Also
removed a bunch of long since dead poop about selection boxes
Added Brush Size Slider + tidied up menu more
Fixed sand's 'friction' setting again
Rejigged a bit so everything isn't completely broken
Slightly improved circular brush algo