[FIX] Vizrank: Properly shutdown/wait when parent deleted

[ales-erjavec]

Issue

File -> Scatter Plot

Run the vizrank dialog (on fairly large data), while the vizrank still runs delete the scatter plot from the canvas. The following error occurs:

--------------------------- RuntimeError Exception ----------------------------
Traceback (most recent call last):
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/visualize/utils/__init__.py", line 169, in <lambda>
    vizrank.selectionChanged.connect(lambda args: set_attr_callback(*args))
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/visualize/owscatterplot.py", line 371, in set_attr
    self.attr_x, self.attr_y = attr_x, attr_y
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/gui.py", line 182, in __setattr__
    callback(value)
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/gui.py", line 2361, in __call__
    self.action(*args)
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/gui.py", line 2429, in action
    self.control.setCurrentIndex(self.model.indexOf(value))
RuntimeError: wrapped C/C++ object of type OrangeComboBox has been deleted

Description of changes

Wait for for completion when the widget is deleted.

Includes

• Code changes
• Tests
• Documentation