Currently, GadTools does not process keyboard equivalents for gadgets.  It
is up to the application writer to implement the correct behavior,
normally by calling GT_SetGadgetAttrs() on the appropriate gadget.  For
some kinds of gadget, the behavior should be the same regardless of
whether the keyboard equivalent was pressed with or without the shift key.
For other gadgets, shifted and unshifted keystrokes will have different,
usually opposite, effects.

Here is the correct behavior for keyboard equivalents for each kind of
GadTools gadget:

Button Gadgets
    The keyboard equivalent should invoke the same function that clicking
    on the gadget does.  There is currently no way to highlight the
    button visuals programmatically when accessing the button through a
    keyboard equivalent.

Text-Entry and Number-Entry Gadgets
    The keyboard equivalent should activate the gadget so the user may
    type into it.  Use Intuition's ActivateGadget() call.

Checkbox Gadgets
    The keyboard equivalent should toggle the state of the checkbox.  Use
    GT_SetGadgetAttrs() and the GTCB_Checked tag.

Mutually-Exclusive Gadgets
    The unshifted keystroke should activate the next choice, wrapping
    around from the last to the first.  The shifted keystroke should
    activate the previous choice, wrapping around from the first to the
    last.  Use GT_SetGadgetAttrs() and the GTMX_Active tag.

Cycle Gadgets
    The unshifted keystroke should activate the next choice, wrapping
    around from the last to the first.  The shifted keystroke should
    activate the previous choice, wrapping around from the first to the
    last.  Use GT_SetGadgetAttrs() and the GTCY_Active tag.

Slider Gadgets
    The unshifted keystroke should increase the slider's level by one,
    stopping at the maximum, while the shifted keystroke should decrease
    the level by one, stopping at the minimum.  Use GT_SetGadgetAttrs()
    and the GTSL_Level tag.

Scroller Gadgets
    The unshifted keystroke should increase the scroller's top by one,
    stopping at the maximum, while the shifted keystroke should decrease
    the scroller's top by one, stopping at the minimum.  Use
    GT_SetGadgetAttrs() and the GTSC_Top tag.

Listview Gadgets
    The unshifted keystroke should cause the next entry in the list to
    become the selected one, stopping at the last entry, while the
    shifted keystroke should cause the previous entry in the list to
    become the selected one, stopping at the first entry.  Use
    GT_SetGadgetAttrs() and the GTLV_Top and GTLV_Selected tags.

Palette Gadgets
    The unshifted keystroke should select the next color, wrapping around
    from the last to the first.  The shifted keystroke should activate
    the previous color, wrapping around from the first to the last.  Use
    GT_SetGadgetAttrs() and the GTPA_Color tag.

Text-Display and Number-Display Gadgets
    These kinds of GadTools gadget have no keyboard equivalents since
    they are not selectable.

Generic Gadgets
    Define appropriate keyboard functions based on the kinds of keyboard
    behavior defined for other GadTools kinds.