Sprite Frame Close External Editor Concurrent Modification Fix
Changes the cleanup loop which stops all of the external subimage
editors to remove the editors using an iterator first. This gets rid of
the concurrent modification exception when you have edited more than
one subimage at the time you save or close the frame. I chose to do it
this way because I didn't want to copy the entire collection. Later the
editors map could possibly be changed to a WeakHashMap which would
automatically remove the keys when the editors are no longer reachable
and eligible for garbage collection. It could also be changed to a
ConcurrentMap which would make it thread safe too.
