Add isOnscreenKeyboardVisisble to Input. Fix closing keyboard in iOS

* Adds isOnscreenKeyboardVisible to the Input Interface
* Creates getter for UITextField on iOS for customization of the keyboard
* Adds method `setKeyboardCloseOnReturnKey (boolean shouldClose)` to IOSInput
* Adds boolean entry `keyboardCloseOnReturn` in `IOSApplicationConfiguration`
* Throws keyDown and keyTyped event for the shouldClose event


Related PR #2211