tabpanel

The tabpanel is a container panel.

Maybe it should be core, currently it's just a widget.

It's proved possible to write it using Baby X as is, but it's thrown up a few issues.

Baby X doesn't maintain a list of children of panels. That was to allow mix and
matching between Baby and non-Baby X widgets. But a container needs such a list
to hide children.
There's also no show / hide. A workaround is to move the child widgets to outside of the
panel. But that's not ideal, as it might trigger unnecessary redrawing.

The whole idea of setting the canvas in the layout is clumsy. I think maybe the canvas
should be changed to reallocate its rgba buffer automatically on resize.

There's no way of drawing to a panel, so you have to set child canvases just to draw lines
round the container.