Replace use of CGRect origin/size with minX/maxX/etc.

[vicng]

For simplicity/readability, we should use the Swift extensions for accessing properties of CGRect's, instead of using their raw form.

eg.
view.frame.minX instead of view.frame.origin.x
view.frame.maxX instead of view.frame.origin.x + view.frame.size.width
view.frame.width instead of view.frame.size.width