Refactor UIKit polluters

Current, there are two ways `UI*` wrappers pollute their respective UIKit class.  This patch refactors those wrappers to use just a single approach -- a `UI*Wrapper` module that's included into the class from `motion/ui/pollute.rb`.

This patch has **no behavior changes** and **maintains backward-compatibility**.

Eventually, I'll move the `UI*Wrapper` classes into the `BW` namespace.  Unfortunately, I'm getting the dreaded **stack level too deep** error when I try right now.  Here's to hoping  RubyMotion v1.36 is released this week.  :)