Improve handling of search fields.

Search fields are hard to work with since their labels are nil.  This
implements two changes to simplify that.

1. Passing `nil` into *any* accessibility label parameter should work
if the element's `accessibilityLabel` is also `nil`.
2. Add `waitForFirstResponderWithAccessibilityLabel:traits:`

This way, search field text can be accessed with a
`nil/UIAccessibilityTraitSearchField` combo.

Related to issue #257.