OWLoadBase widget

[janezd]

We would need a common base for load widgets because:

• widgets for loading files have a lot of replicated code,
• some are implemented better, some worse,
• and as with all replicated code, improving one widget does not improve others.

When implementing this

• the code should, I suppose, be taken from the CSV widget, which is by far the most systematic load widget,

• perhaps see OWSaveBase for inspiration, because it seems to do the job well; when there are no special functionalities, save widgets need only two methods, like

  @Inputs.model
  def set_model(self, model):
      self.data = model
      self.on_new_input()
  
  def do_save(self):
      with open(self.filename, "wb") as f:
          pickle.dump(self.data, f)