Sent when an accessibility client requests the keyboard shortcut
of the control, or the keyboard shortcut of a child of the control.
A keyboard shortcut can either be a mnemonic, or an accelerator.
As a general rule, if the control or child can receive keyboard focus,
then you should expose its mnemonic, and if it cannot receive keyboard
focus, then you should expose its accelerator.
Return the keyboard shortcut string of the control or specified child
result field of the event object. Returning an
empty string tells the client that the control or child does not
have a keyboard shortcut string, and returning null tells the client
to use the platform keyboard shortcut string.