Remove gensim

[ajdapretnar]

Issue

Remove gensim to avoid installation issues.
Fixes #1127.

Description of changes

• Move gensim's Dictionary into a separate Orange class (copy with some methods removed).
• Fix bag of words to work with sklearn's methods.

Includes

• Code changes
• Tests
• Documentation

[janezd]

I just noticed that the network add-on also requires gensim to compute graph embeddings. To get rid of it there, we'd need to make the graph embeddings widget "optional"? Or is there an alternative library we can use?

[ajdapretnar]

@janezd I see two tentative options:

• replace word2vec with an alternative distribution (https://pypi.org/project/word2vec/) Scrap that, it is completely unmaintained.
• use a different embedder (FastText or alike)
  But I am no expert here. Essentially, we should find a good sequence model, from how I understand it.