An action is a function with three parameters w1 w2 ev, where
w1 is the source widget, w2 the target widget, and ev the event
(Trigger.t) that triggered the action.
Warning, the background type corresponds actually to the Style.t
type, which means it includes color backgrounds, image patterns, corner and
shadow styles.
The type Widget.t is a union of all kinds of widgets: Box, Button, Check box,
Image, Label, Slider, Text display, and Text input, plus the Empty
widget.