Tk automatically creates class bindings for listboxes that give them
the following default behavior:
When button 1 is pressed over a listbox, the element underneath the
mouse cursor is selected. The mouse can be dragged to select a
range of elements.
The ends of the selection can be adjusted by dragging with mouse
button 1 while the shift key is down; this will adjust the end
of the selection that was nearest to the mouse cursor when button
1 was pressed.
The view in the listbox can be adjusted by dragging with mouse button 2.
The behavior of listboxes can be changed by defining new bindings for
individual widgets or by redefining the class bindings.
In addition, the procedure tk_listboxSingleSelect may be
invoked to change listbox behavior so that only a single element
may be selected at once.