Export options dialog

[Lars Clausen <lrclause cs uiuc edu>]

I decided to scratch an itch I've had for a while:  Options for exporting.
I was thinking I'd have to add some hook system, but it turns out to work
just fine inside the export plugins.  I changed the PNG plugin to ask for
image size, and that works just fine in the newest CVS.  I made sure to
factorize and comment the code, so anybody who feel like it can fairly
easily add dialogs to other exporters.

-Lars

-- 
Lars Clausen (http://shasta.cs.uiuc.edu/~lrclause)| Hårdgrim of Numenor
"I do not agree with a word that you say, but I   |----------------------------
will defend to the death your right to say it."   | Where are we going, and
    --Evelyn Beatrice Hall paraphrasing Voltaire  | what's with the handbasket?