New Dia-based documentation

Following a chat with nsh on IRC I decided to have a go at documenting
Conglomerate's internals a bit more.

Since a picture is worth a thousand words, here's the result so far:
http://cvs.gnome.org/viewcvs/conglomerate/doc/devel/Conglomerate-Overview.png?view=markup

There's a Dia file in doc/devel/Conglomerate-Overview.dia
I generated this png file using Dia's File->Export facility.

I believe the link above will always be to the latest version of the png
file; perhaps we could add this link to the Conglomerate website?

So far the diagram covers the outer parts of the API, I'd hoped to cover
the details of CongEditorLineManager/Iter etc but it's getting late - I
plan to do that another day.

Dave