Rework `UISearchBarTextField` first responder detection to support iOS 7

The view hierarchy for `UISearchBar` instances has changed under iOS 7 such that the direct superview of the `UISearchBarTextField` is no longer the `UISearchBar` instance, breaking the first responder detection. This change does an upward search through the view hierarchy to find the parent `UISearchBar`. It is backwards compatible to iOS 5 & 6 while also supporting iOS 7.