title: New property needsInputModeSwitchKey in iOS11
body: <p>On What's New in iOS11 page, I found that 'Added needsInputModeSwitchKey, a property of UIInputViewController to control the display of the input-mode switch key'.The new property needsInputModeSwitchKey is readonly. I have developed a keyboard Extension via Swift, but I have no idea about this new property. Can someone give some advice. Thank you!.</p>