In the `examples/platform_view` example, which demonstrates
transitioning from a Flutter view to a general iOS UIView and back, as
well as using channels to communicate between the two, the Flutter view
renders correctly, but in the iOS UIView, the FAB equivalent button in
the lower-right corner is rendered without the '+' icon. This is because
no binding as declared for the add icon in the storyboard. This adds the
missing binding.

Further, this eliminates the use of the deprecated
[UIButtonTypeRoundedRect](https://developer.apple.com/documentation/uikit/uibuttontype/uibuttontyperoundedrect?language=objc),
falling back to the default instead.

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Co-authored-by: Chris Bracken <chris@bracken.jp>