GUI.pyw: fix an error that seems to happen only on Python 3.5

When closing down the program, this error message appeared: 'TypeError: catching classes that do not inherit from BaseException is not allowed'. It seems to depend on Tkinter's PhotoImage object being destroyed after the root windows's destroy() method is called.

data/tools/GUI.pyw

@@ -1231,8 +1231,10 @@ Icons are taken from the Tango Desktop Project (http://tango.freedesktop.org), a
                               "Do you really want to quit?",
                               icon = WARNING)
             if answer:
+                ICONS.clear()
                 self.parent.destroy()
         else:
+            ICONS.clear()
             self.parent.destroy()
 
 root=Tk()