Create a Font object, like how there is a Rectangle object
Like `Rectangle` I think it might make sense to have a javascript object for `Font`. Right now `text.font` has to be set to a `NSFont` object, exposing the underlying obj-c objects. Also the `systemFontSize` defined as `set` is a bit strange because  then it's usage is `text.systemFontSize = 40`, but this will do more than just set the font size, it will override the font family to the system font, which kinda makes sense from the property name, but I think is still strange ;)
