SE-0307: Allow interchangeable use of CGFloat and Double types

[scanon (Steve Canon)]

@cukr

Wouldn't it make harder to move away from CGFloat, if it starts to be used more? (because people treat it just like a typealias)

No, because it allows new API to use Double and still interoperate with existing API using CGFloat. The current state of things is that we would like new libraries to use Double, but they are de-facto forced to use CGFloat to avoid gratuitous conversions when they interoperate with existing API.