FlxTrail code style and organization

[Geokureli]

Allows more customization of FlxTrail by separating behavior into separate methods, for instance to accomplish #443
is as simple as:

class OffsetTrail extends FlxTrail
{
	/**
	 * An offset applied to the target position whenever a new frame is saved.
	 */
	public final frameOffset = FlxPoint.get();
	
	override function addTrailFrame()
	{
		super.addTrailFrame();
		
		_recentPositions[0].addPoint(frameOffset);
	}
	
	override function destroy()
	{
		super.destroy();
		
		frameOffset.put();
	}
}