Make MouseEventManager compatible with FlxObject, closes #823

Allowing to use this class only on `FlxSprite` was kind of an arbitrary restriction, as the hitbox is added at `FlxObject` level. 

This includes the following API changes:

```
addSprite() -> add()
removeSprite() -> remove()
setSpriteMouseChildren() -> setObjectMouseChildren()
isSpriteMouseChildren() -> isObjectMouseChildren()
setSpriteMouseEnabled() -> setObjectMouseEnabled()
isSpriteMouseEnabled() -> isObjectMouseEnabled()
```

Lots of type params. I have a feeling @crazysam is not gonna like this. :P They make sense though, otherwise you can't properly type the callback functions.

Also, might have found a bug in hxcpp.. This won't compile to cpp with `FlxBasic.get_cameras()` being inlined... :/