fix: Add itemChange method to AttackerItem to handle scene removal

[sandorstormen]

Cursor-produced fix for the bug of #21.

[mrkickling]

I still get the error on your branch.

1. Create attacker in a fresh MAL GUI window
2. Load another scenario which deletes the old scene and creates a new one

RuntimeError: Internal C++ object (AttackerItem) already deleted.
Traceback (most recent call last):
  File "/home/mrkickling/Documents/github/mal-gui/mal_gui/object_explorer/attacker_item.py", line 48, in update_status_color
    if self.scene() is None:

[mrkickling]

Found a fix (worked for me now at least). Thanks ChatGPT.