Made it return True if the release is handled by the base class

nion/swift/ImageCanvasItem.py

@@ -1227,7 +1227,7 @@ def mouse_pressed(self, x: int, y: int, modifiers: UserInterface.KeyboardModifie
 
     def mouse_released(self, x: int, y: int, modifiers: UserInterface.KeyboardModifiers) -> bool:
         if super().mouse_released(x, y, modifiers):
-            return
+            return True
 
         delegate = self.delegate
         widget_mapping = self.mouse_mapping