New debug plugin

I've created src/plugin-debug.c

This is a plugin intended purely for debugging; you have to enable it
with --enable-debug-plugin (this is done by default in autogen.sh for
people building from CVS).

It adds a new "Debug: Dump Area Tree" item to the Tools menu (none of
the strings get or need translations as this is purely for debugging). 
When you select this it dumps the CongEditorArea tree into an XML file
and opens it in a new window.  

This should be really useful for debugging problems with the new editor
widget; I'm already seeing obvious mistakes in my code now that there's
a good way to visualise the data structure.  If you're want to hack on
Conglomerate I recommend trying out this feature!

Ideas and patches for other debug options welcome...

Dave

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