Some cleanup
- removed Vector2D, wasn't used anywhere
- removed IntPt and replace references to it in U.hx by FlxPoint
- move interfaces to a interfaces subfolder
- truncated all imports
- removed all randomness-related functions from U.hx, FlxRandom convers all that functionality and should be used instead because it's deterministic
- removed U.arrayContains(), there's indexOf()
- removed FlxUIAssets.init(), wasn't used