Every button has a button type, which defines default values for the button’s properties. Button types are arranged in a hierarchy, with specialized types inheriting from more general types, so that it’s easy to define special-purpose types of buttons for specific tasks.
Define a button type called name (a symbol).
The remaining arguments
form a sequence of property value pairs, specifying default
property values for buttons with this type (a button’s type may be set
by giving it a type property when creating the button, using
the :type keyword argument).
In addition, the keyword argument :supertype may be used to
specify a button-type from which name inherits its default
property values. Note that this inheritance happens only when
name is defined; subsequent changes to a supertype are not
reflected in its subtypes.
Using define-button-type to define default properties for
buttons is not necessary—buttons without any specified type use the
built-in button-type button—but it is encouraged, since
doing so usually makes the resulting code clearer and more efficient.