2nd try to fix macOS X blank screen when leaving fullscreen editor mode

See issue olivierkes#24.

The first attempt to fix this problem was with commit:

    Try to fix macOS X blank screen when leaving editor fullscreen mode
    1ae0a77

manuskript/ui/editors/fullScreenEditor.py

@@ -123,6 +123,7 @@ def __init__(self, index, parent=None):
     def __del__(self):
         # print("Leaving fullScreenEditor via Destructor event", flush=True)
         self.showNormal()
+        self.close()
 
     def setLocked(self, val):
         self._locked = val