Use gyb from the Swift project for the AppKit/UIKit extensions

[liscio]

The MutableProperty wrappers are effectively copy & pasted with a few tweaks here & there for types, associated keys, and so on. It would be wise to figure out a way to use the gyb (Generate Your Boilerplate) tool from the Swift project to help us overcome a lot of that (sadly necessary) boilerplate.

[neilpa]

Good idea - https://github.com/apple/swift/blob/master/utils/gyb.py

[andersio]

Implemented in #142 #147. :)

[dmcrodrigues]

@andersio In my humble opinion, that should have been done in a separate PR and not in a branch expected to restructure/rename a lot of things and which may not be merged soon. I think this won't make our life easier because it will require a synchronization between current version targeting Swift 2.x and the new one targeting Swift 3.

[andersio]

@dmcrodrigues They are now split, and I have setup the RAC5 as the branch for reviewed PR like the main repo.