main changes are:
- Modified marker (* character before name) goes up the project tree by
marking all parents of a modified object as modified.
- Impact management when changing an object's ID. Allows for some
refactoring to occur, while marking the impacted elements as modified,
or aven creating "altered" versions if imapcted element was only in game
source.
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.
