When porting from the old client ID to the new client ID, here in ``handleClientIdChange``, we were taking in a brand new defaults object, then seeing if the old ID was non-empty. If it's not empty, we then set the old pref keys into the new prefs object (which again matches the default properties passed initially to ``PreferencesManager.getPreferenceStorage(module, defaultPrefs``

However, we were passing ``append==false`` to ``setAllValues()``. What this did was it cleared out all the new properties defined in the defaults object and only wrote the properties that existed for the old clientID.

With this change to ``append==true``, we take the new defaults object and merge in the old existing pref keys if they exist. This is the behavior that we want and I'm pretty sure the original changes were unintended. Let me know if I've missed something.

This fixes the preferences migration issues I was seeing in @RaymondLim's pull #3097 for word wrap and line numbers.