Fitter preprocessors

[lanzagar]

Orange version

3.4.0

Expected behavior

Learners use preprocessors.

Actual behavior

Learners extending the Fitter base class do not use preprocessors.

Steps to reproduce the behavior

Use a learner on e.g. hearth_disease data set

Additional info (worksheets, data, screenshots, ...)

[lanzagar]

This is a very high priority bug in the latest version that needs a fix.
@pavlin-policar do you have time to check this out as you are most familiar with the code?

[pavlin-policar]

Sure thing, I'll get right on it.

[pavlin-policar]

Should be fixed with #2093