Introduce FlxAngle, get rid of FlxPTMath

- added a new `FlxAngle` class contaning a bunch of functions from `FlxMath` & `FlxPTMath` related to angle calculations in some way. Moved `RAD` and `DEG` over from `FlxG` (didn't really belong there).

- moved the remaining functions in `FlxPTMath` to `FlxMath`:
 - `maxAdd()`
 - `wrapValue()`
 - `dotProduct()`

- also removed `FlxMath.degreesToRadians()` (not really needed anymore due to `FlxAngle.asDegrees()` and `FlxAngle.asRadians()` - same for vars in `FlxPTMath` that were equivalent to `DEG` and `RAD` - this made it possible to get rid of `FlxPTMath` entirely, which is good because of possible naming confusions