Treema fix

I was out hunting Treema bugs.
Found just the one: one shouldn't be able to switch music files from a readOnly Treema.

Extra note: The Config Schema of a System as can be accessed from the Level Editor is editable while nothing else is. I wasn't sure whether this was a feature, a coding bug where readOnly wasn't set or a Treema extension bug. Could someone please get back to me on this?