ImageStack.urlSelectionTableIsVisible might return wrong result

[t20100]

Issue when the widget or one of the ancestor is not visible: isVisible() is one of the property which is not symmetric to setVisible(). As result it can be false is some cases for false reason.

The following code from ImageStack`looks to have the same problem

    def urlSelectionTableIsVisible(self):
        return self._urlsTable.isVisible()

Initially raised here: #3742 (comment)

This needs to be checked.