Add revision description in `entity.set_params`

Description of the limitation and why it is relevant to address

As a developer I want to be able to control the description of a revision i create with the api module such that it clear when looking into the revisions what happened

I think this is relevant for the VIKTOR platform because …

Submitter proposed design (optional)

entity.set_params(params, description)

Current workarounds

using REST api instead of viktor api module

requested by @wouterhouzr