Button gadgets (BUTTON_KIND) are perhaps the simplest kind of GadTools
gadget.  Button gadgets may be used for objects like the "OK" and "Cancel"
buttons in requesters.  GadTools will create a hit-select button with a
raised bevelled border.  The label supplied will be centered on the
button's face.  Since the label is not clipped, be sure that the gadget is
large enough to contain the text supplied.

Button gadgets recognize only one tag:

GA_Disabled (BOOL)
    Set this attribute to TRUE to disable or ghost the button gadget, to
    FALSE otherwise.  The default is FALSE. (Create and set.)

When the user selects a button gadget, the program will receive an
idcmp_gadgetup event.

If clicking on a button causes a requester to appear, for example a button
that brings up a color requester, then the button text should end in
ellipsis (...), as in "Quit..."