For supported windows (currently the transaction entry window, and list windows), adds an icon to the toolbar. The icon name will be the same as the handler name. When clicked, the handler will be called with the windowRef passed as a parameter. Returns true (1) on success, 0 if no icon was added (due to unsupported window, or toolbar full). Your handler does not need to be declared public since the script context is known at the time of installation.