UIKitBackend: Implement toggle button and checkbox polyfills

[stackotter]

UIKitBackend doesn't support ToggleStyle.button or ToggleStyle.checkbox because UIKit doesn't have built-in controls for those. It'd be good to support them anyway, so we should implement some custom UIViews that emulate how we think toggle buttons or checkboxes should work on iOS.

[bbrk24]

On iOS, the checkbox style could take inspiration from these list-like pickers found in system apps like settings:

Not sure if tvOS has an equivalent.