Feature: support widget instance as Frame focus part in constructor

[penguinolog]

• separate frame tests

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 7973065026

Details

• -1 of 9 (88.89%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 71.298%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
urwid/widget/frame.py	8	9	88.89%

Totals
Change from base Build 7973061098:	0.01%
Covered Lines:	8938
Relevant Lines:	12642

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💛 - Coveralls