The ResizeButton widget provides a resizeable button widget for use
in an HList column header. When placed in the column header, the
edge of the widget can be selected and dragged to a new location to
change the size of the HList column. When resizing the column, a
column bar will also be placed over the HList indicating the eventual
size of the HList column. A command can also be bound to the button
to do things like sorting the column.