CopyOnWriteArrayList, to allow listeners to unregister themselves due to
an event while preventing ConcurrentModificationExceptions.
Modified all GameDataElement.elementChanged concrete implementation to
remove the backlink from the oldOne.An element pointed by an altered
element will not show the game source element in its backlink list
anymore.
!! Also, "created" elements show the "unsaved" asterisk in their
description until they're saved to disk.
Added a button to the spritesheet editor to open image in an external
tool too. This may or may not remain in the app.
