Simple GTK+ 3.0 container subclass example

[Mark Leisher <mleisher math nmsu edu>]

This is a relatively simple example of how to subclass the Container 
widget in GTK+ 3.0.

  http://www.math.nmsu.edu/~mleisher/Software/other/labcon.zip

Once upon a time, I needed a container with the following capabilities:

1. It holds one child widget.
2. It places a label on the left or right side of the child widget.
3. The label can be text or a Pixbuf, and the label can be aligned to 
the right, center, or left.
4. All of the labels in a group of these containers needed to have the 
same width.

I was surprised how long it took me to get this working in GTK+ 2.*. I 
had to dig through the sources of a lot of different programs before I 
finally narrowed the code down to the minimum necessary for a simple 
container subclass.

Maybe this will be useful to someone out there developing custom widgets.

Yes, there are other ways to do this, but that's not the point.
--
Mark Leisher