Make Cocoa view sources non-generic

While the design in #448 is far-and-away better from a code perspective, it
doesn't work because Cocoa objects can't be generic (i.e. anything that
exposes an interface to Objective C). Peep
http://docs.xamarin.com/guides/ios/advanced_topics/limitations for the
details.
Merge pull request #545 from reactiveui/cocoa-y-u-no-like-generics

Make Cocoa view sources non-generic