Teaching application: need guidance

[Franck Martin <franck sopac org>]

I would like to develop a very simple application, but I'd like to get 
some pointers on how it should be done. I will do it with glade and anjuta.

The application is simple, when you click on an icon in a toolbar it 
will bring a transparent layer over the whole screen upon which you will 
be able to draw.

The purpose, you bring a spreadsheet on your screen, project your screen 
on the wall and then you can highlight some areas with your pen/mouse....

When you click on an icon then all the action on the screen are passed 
to the applications behind the layer...

How do you i do a transparent layer (style gnome canvas)?

Cheers
Franck