`org-export-export-preprocess-string' was a dummy copy of
`org-export-preprocess-string', adapted for specific needs
of org-mw-export. We now use `org-export-preprocess-string'
directly. This requires a minor change in this latter function:
if :for-backend is not explicitely passed, then the let binding
of `org-export-current-backend' falls back on the previous value
of org-export-current-backend'.
This only lead to a contributed file, this is neither robust nor fast.
But maybe throwing the existing code into a contrib file
will make someone come up with an idea.
In this file I will implement interactive modifications of tags/todo
queries. This is code from Christopher League which still needs some
work before it will become either CONTRIB or even core.
