It only passes the OPF file and sets up the watcher for all xhtml files if
the external editor is named "pageedit", now. Otherwise, it only passes the current xhtml file to other programs. We had to change that because the earlier approach basically limited the external editor button to be used for PageEdit. The couldn't edit xhtml files with a different xhtml editor assigned to the button.
Also keep in mind that the button can be assigned to a batch file that could do all kinds of theoretical processing before passing the xhtml file in question to an external editor.