Merge FlxPTColor into FlxColor and move FlxVelocity

Integrated `FlxVelocity` into the util package and moved some functions it cotained that are more fitting in other util classes:

- org.flixel.plugin.photonstorm.FlxVelocity -> org.flixel.util.FlxVelocity
 - `FlxVelocity.distanceBetween()` -> `FlxMath.distanceBetween()` 
 - `FlxVelocity.distanceToPoint()` -> `FlxMath.distanceToPoint()` 
 - `FlxVelocity.distanceToMouse()` -> `FlxMath.distanceToMouse()` 
 - `FlxVelocity.distanceToTouch()` -> `FlxMath.distanceToTouch()` 
 - `FlxVelocity.angleBetween()` -> `FlxAngle.angleBetween()` 
 - `FlxVelocity.angleBetweenPoint()` -> `FlxAngle.angleBetweenPoint()` 
 - `FlxVelocity.angleBetweenMouse()` -> `FlxAngle.angleBetweenMouse()` 
 - `FlxVelocity.angleBetweenTouch()` -> `FlxAngle.angleBetweenTouch()` 

Also completely merged `FlxPTColor` into `FlxColor` now.