Refactoring: move monitored_list module to the widgets package

[penguinolog]

This is a critical element for the container widgets.
All classes are exported via urwid namespace without changes.

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

[coveralls]

Pull Request Test Coverage Report for Build 8003854322

Details

• 0 of 5 (100.0%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 72.508%

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Totals
Change from base Build 8003785035:	0.0%
Covered Lines:	9137
Relevant Lines:	12666

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💛 - Coveralls