feat(macOS): native buttons (NSButton)

[zbaylin]

This does a couple main things:

• creates a wrapper for the NSView class similar to that created for NSObject in #1023
• creates a Native.Input.Button module that houses the basic functions to render a button
• wraps those functions in a standardized Node (similar to an ImageNode)
• wraps the NativeButtonNode in a NativeButton nativeComponent
• adds a cleanup function to the Node class
  • this is to deal with removing the NSButtons from the view once they are removed. Before, when a child was removed, setParent(None) was called on the child. However, if you have a basic view hierarchy like Root -> Example -> [Buttons] and the Example is removed, we have to notify all the children that they have been removed too.

[zbaylin]

After further investigation, here are the things left that don't (and won't, on macOS at least) work:

• background color -- Cocoa explicitly disallows setting the background color of a "bordered" button, i.e. one that has any styling: https://developer.apple.com/documentation/appkit/nsbuttoncell/1529743-backgroundcolor?language=objc
• opacity -- buttons can either be fully opaque or fully transparent, no in-between: https://developer.apple.com/documentation/appkit/nsbuttoncell/1533667-opaque?language=objc
• height -- the height of the button itself cannot be changed, only the frame which it occupies. Again, this is a styling issue (I assume it's because they use bitmaps to construct the buttons and it only scales well horizontally)
• rotational transformations -- I might be able to get this to work but it would take some complex math. If anything, I think it would make sense to be in a separate PR.

@bryphe let me know if I need to change anything else, or if you have any questions about anything I have already!