Fix filter filter regex

Now the external material file dialog shows the file types defined in the
template again. Before this fix it did only show "All files (*)".
I don't know what the old syntax was supposed to do or whether it ever worked
with boost::regex, but with std::regex the new one is correct.

src/frontends/qt4/qt_helpers.cpp

@@ -556,7 +556,7 @@ FileFilterList::FileFilterList(docstring const & qt_style_filter)
 
 		// Everything from the start of the input to
 		// the start of the match.
-		parse_filter(string(what[-1].first, what[-1].second));
+		parse_filter(string(it, what[0].first));
 
 		// Increment the iterator to the end of the match.
 		it += distance(it, what[0].second);