Fix #648: Verify that `inputToolbar` `isFirstResponder` before responding to `UIKeyboard` notifications

This PR fixes #648 by checking verifying that the `inputToolbar` `textView` `isFirstResponder` - OR whether it is already 'in motion', i.e. in the process of being hidden AFTER it `resignFirstResponder` is called.

**NOTE:** The changes introduced in #646 will also need this fix - i.e. the same conditional must be applied to the 'keyboard did hide' notification handler.
hey @toblerpwn - aren't these 2 conditions mutually *inclusive*?

that is, if we are the firstResponder, then the layoutGuide will be non-zero. likewise, if the layoutGuide is non-zero, then we will not be the first responder.