Update actions moveBy, rotateBy, and scaleBy to use relative positioning

[eonarheim]

Context

Currently the actions moveBy, rotateBy, and scaleBy are confusing, they move a certain quantity in a certain time. They should instead move a quantity relative to the current quantity, this is more expected.

Proposal

The method signature should also hint that things are moving by an offset moveBy(offsetX: number, offsetY: number, pixelsPerSecond: number)

moveBy(10, 10, 2) moves and actor 10 pixels to the right, and 10 pixels down at 2 pixels per second.
rotateBy(Math.PI/2, .2) rotates the actor clockwise PI/2 radians at .2 radians per second
scaleBy(1, 1, .5, .5) scales the actor 100% in the x and 100% in the y at .5 scale units per second