Support COMMENT for tasks

[sunkup]

Purpose

Support task comments.

Short description

• Added a comment field to Task.
• Enabled Task to read the COMMENT property into the comment field
• Enabled Task to generate the COMMENT property from the comment field
• DmfsTask: Generates COMMENT data row (probably with language null) from Task.comment field
• DmfsTask: Processes COMMENT data row and fills Task.comment field from it

Checklist

• [X] The PR has a proper title, description and label.
• [x] I have self-reviewed the PR.
• [ ] ~I have added documentation to complex functions and functions that can be used by other modules.~
• [X] I have added reasonable tests or consciously decided to not add tests.