Feedback for buttons

The program Feedback.m3 illustrates a simple VBTkit switch with two different kinds of feedback. It displays two buttons side by side with bordered feedback and shadowed feedback, respectively. The appearance changes as the user clicks and releases a mouse button in the button. This is shown in following images: The first picture shows the initial appearance when no buttons are being pushed. The middle picture shows the appearance when the button labeled ``Push1'' is pressed. The last picture shows the appearance when ``Push2'' is pressed.

In the normal state the bordered feedback draws a border (a shadow of style ``raised'') around the button---the shadowed feedback displays nothing. In the excited state (when the button is pressed) the bordered feedback draws a shadow around the button of style ``lowered''---the shadowed feedback displays a plain border. When the button is released the buttons regain their normal appearance.

A feedback (Feedback.T) maintains a boolean state: ``on'' or ``off.'' The appearance of the feedback depends on this state.