notify-on-change

Attaches a callback to the style list. The callback is invoked whenever a style is modified.

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 the changing style after a set of styles has been processed.

• (send a-wx:style-list notify-on-change f) -> small integer
      f : a procedure of one argument: a wx:style% object or null