Add levelBounds and fix sprite flash glitch.

Units and weapons now handle level boundaries based on _levelBounds
attribute of each level (which can be assigned by defining LevelWidth and
LevelHeight as attributes under a Level element in LevelData.xml).
This should set the stage for implementing levels larger than the screen
and a camera class.

Sprites now flash properly and set color to grey when disabled.
Add ScrollUp/ScrollDown to InputManager.

Use these to detect scroll events for the mouse.
Change the constant WHEEL_UNITS_PER_SCROLL to adjust scroll sensitivity.

Rocket is broken based on adding velocity of user to projectile (added for
flamethrower). Need to have projectiles accelerate in direction of
original angle, not current Velocity.
Fix Rocket fire direction bug
Radial bar allows negative angle difference.

You can now make a radial bar decrease in the opposite direction by
specifying a start angle greater than the end angle.