OWLoadBase widget

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

```python
@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)
```

Compare the CSV widget's loading implementation with OWSaveBase, which the issue identifies as an example of a common base. Define the shared behavior and identify the load widgets with duplicated code; done means they use the common base without losing their specialized functionality.

Written by the indexing model from the issue text.