Fix reference to setPlaceholderTextNode

This is now an instance method, whereas before it was global. Annoyingly, we
need to access this from from within fixCursor which has no reference to the
RTE instance itself (and it would be a pain to pass one down). For now, just
referring to the global `editor` variable if it exists (i.e. if the script
loaded in an iframe). Need a better solution longer term though.