callback
The command procedure to be executed when a sub-window is interactively selected via one of its tab-buttons. This is invoked only on an interactive button click, not when a sub-window is programmatically selected by setting its visibility.
The window identifier of the tab window is passed to the procedure as parameter 1. The sub-window (also a window identifier) is passed as parameter 2.
New tab windows have a .select procedure of null.
<window>.select(<window>,<sub-window>)