The SGH_KEY command indicates that the user has pressed a key while the
gadget is active.  This may be any key, including non-character keys such
as Shift, Ctrl and Alt.  Repeat keys (one call per repeat) and the Amiga
keys also cause the hook to be called with the SGH_KEY command.  The hook
is not called for "key up" events.

The SGH_KEY command must be supported by any custom string editing hook.
There are no parameters following the SGH_KEY command longword.  All
information on the event must be derived from the SGWork structure.

Intuition has already processed the event and filled-in the SGWork
structure before calling the hook.  The information included in this
structure includes the type of action taken (EditOp), the new cursor
position (BufferPos), the new value in the buffer (WorkBuffer),  the
previous value in the buffer (PrevBuffer), the input event that caused
this call (IEvent) and more.