Embedding one app within another

[Matthew <matthew mattshouse com>]

Is there a way with stock gtk to embed one application within another?  Both
apps are being developed by myself, so I'm not opposed to adding this
capability if mods have to be made to the code.  I seem to recall reading about
a container widget that allows this, but I can't seem to find it now.

For example, I want to run one app within a notebook widget of another.

Matthew