callback
The command procedure to be executed when the mouse is dragged within the sub-panel. The window parent of panel and panel itself are passed to the procedure as parameters. When panel belongs to a table window, the row/column index of the panel is passed as a third parameter.
This element is only available to panels of type dial, slider and list. For dial and slider panels, the procedure is executed repeatedly on every change of mouse position while within the sub-panel. For a list panel, the procedure is executed whenever a new option is selected at the end of a mouse drag or on a single click.
Initially, panels have a .drag procedure of null.
panel.drag (<window>,<panel>{,<row>})
win1.panels[2].drag = { args w=window, p=panel tell <'New dial value:',p.value> }