Attaches a callback to the style list. The callback is invoked whenever a style is modified.
The form of a wxStyleNotifyFunc is:
typedef void (*wxStyleNotifyFunc) (wxStyle *changedStyle, void *data)
The data pointer is passed on to the wxStyleNotifyFunc function when it is invoked. Often, a change in one style will trigger a change in several other derived styles; to allow clients to handle all the changes in a batch, NULL is passed in as changedStyle after a set of styles has been changed.