Add FlxSwipe and FlxG.swipes

A FlxSwipe has the following public fields: ID, startPosition, endPosition, distance, angle, duration
The ID is either the touchPointID of a touch or FlxMouseButton.LEFT, MIDDLE or RIGHT

Also includes a small cleanup for FlxTouch
Initialize FlxG.swipes at its declaration
FlxMouse.destroy(): Add missing FLX_NO_MOUSE_ADVANCED conditional
FlxTouch: forgot to initialize _justPressedPosition
FlxMouseButton: Change button IDs to avoid conflicts with touch IDs
Merge pull request #848 from Gama11/swipes

Add FlxSwipe and FlxG.swipes