Implemented by subclasses to respond to mnemonics.
A string representing the mnemonic to handle.
If the view’s mnemonic is the same as the characters of the string aString, the view should take the appropriate action and return YES. Otherwise, it should return the result of invoking super’s implementation. The default implementation of this method simply passes the message down the view hierarchy (from superviews to subviews) and returns NO if none of the view’s subviews responds YES. Mnemonics are not supported in macOS.