Fix regression: Columns render incorrect BOX/FLOW widgets height

[penguinolog]

• Fix and re-sort calculation
• Add tests to cover regression
• Add helper property decoded_text to the Canvas to stop a copy-paste decoded lines collection for tests

Fix: #753

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 7567814892

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.002%) to 69.896%

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Totals
Change from base Build 7560036848:	-0.002%
Covered Lines:	8364
Relevant Lines:	12040

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💛 - Coveralls