Added velocity parameters and associated functions to actors

[Mambouna]

Since many game objects move along straight paths with a constant speed, this PR adds velocity parameters to actors as well as a function that simply moves the actor by its velocity and one to intersect straight-line-movement.

• actor.vx and actor.vy are the velocity components in either axis, analogous to actor.x and actor.y.
• actor.vel is the combined velocity as a tuple, analogous to actor.pos.
• All can be read and set independently with no issues.
• actor.move_by_vel() moves the actor by its velocity. If called every update(), this makes for very easy straight-line-movement
• actor.move_by_vel(2.0) will double the speed. The optional scale parameter allows for things like slowmotion with straight-line-movement.
• actor.intersection_velocity(target, speed) will calculate a velocity tuple of the given magnitude to intercept the target actor as long as its velocity remains constant. If no valid interception exists, None is returned.