ignore duplicate quit events when the window is closed twice

quit_event ran its cleanup path again when a second delete-event
arrived after the state had already switched to Quit_State, hanging
the application on shutdown. Return early instead.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/damaris/gui/DamarisGUI.py

@@ -333,6 +333,9 @@ class DamarisGUI:
         """
         expecting quit event for main application
         """
+        if self.state == DamarisGUI.Quit_State:
+            # quit already handled, e.g. window closed twice
+            return True
         if self.state in [ DamarisGUI.Edit_State, DamarisGUI.Quit_State ]:
             self.state = DamarisGUI.Quit_State
             # do a cleanup...