Adding a handle bar to a vbox or hbox

[Grant Likely <likelyg cadvision com>]

Is there a way to add a handle bar (terminology?) to a hbox or vbox
between two sizeable widgets so that the user of a program can change
the size of widgets without resizing the window.  An example of this
would be a text editor that allows you to split the pane so that you
have two views at different points in the same file, and the bar between

the two views can be moved up and down.

Any help would be appreciated, I haven't been able to find anything in
the documentation.

Thanks,
G.