Massive update: The Gimp -> GIMP, other changes for 2.4

In case you are surprised by the large number of changes when you do your
"svn update", here are some comments...

We still had many files using "The Gimp" instead of "GIMP". I spent most
of the day going through all tutorials and other files, making the
necessary replacements and making other minor updates. I also made
several updates to the pages in order to prepare the release of 2.4
(which will be the stable version by the time the new site goes live).

I also deleted all .cvsignore files that are obsolete since we moved from
CVS to SVN. For SVN, you have to do the following if you want to edit
the list of ignored files:
svn propedit svn:ignore <dirname>
where <dirname> is the directory containing the files to be ignored.
Remember to do that whenever you add a new *.htrw file, so that the
corresponding *.html file does not generate SVN warnings. You may want
to set your $EDITOR environment variable before running svn propedit.

It would be nice to have volunteers to update the release notes...

-Raphaël