OWPreprocess Text: add option to filter on POS tags

[ajdapretnar]

Issue

Implements #678.

Description of changes

Add option to filter POS tags directly in Preprocess Text.
Add setter and getter to Corpus for easier pos_tag handling.

Includes

• Code changes
• Tests
• Documentation

[ajdapretnar]

One possible improvement: map existing POS tags to dict and offer them as a dropdown.
Issue: Penn Tree Bank and conllu have different tag sets. Penn is more detailed (i.e. JJR, JJS...), while conllu is simpler in the first form (VERB, NOUN, ADJ) and has specific tag information "hidden" (accessible in parsed tokens, which we currently don't support).

[PrimozGodec]

It looks good to me. I would say we keep the implementation as it is now and later (when we also add more POS taggers) we can implement it as a dropdown of options.