Smart quotes patch

I have attached a diff to fix/implement smart quotes. I used svn
diff, so if I did it wrong, please let me know.

All I did was change rule #7 (see fl_DocLayout.cpp) so that quotes in
isolation are changed to opening quotes.

I added GUI support for GTK+. I could do Windows myself, but I would
need help for Cocoa and anything else that is currently supported.

I am willing to do more, but I wanted to see if smart quotes would be
rejected outright. I think it works well, but I don't know how the
regular developers feel.

In particular, I was thinking of:
* An option to always convert backtick to left-single-quote and
apostrophe to right-single-quote. This would allow easy manual entry
of smart quotes which are tricky because a word can sometimes start
with an apostrophe (like, "go get 'em")
* Allow smart quotes to be turned off just for single-quotes (like OOo
and Word).
* Add a toolbar button to toggle between smart quotes and ASCII quotes.

Bobby