Expose widget and event_loop packages

[penguinolog]

urwid.widget expose classes and methods,
which are not public directly:

• WidgetContainerListContentsMixin
• calculate_left_right_padding
• calculate_top_bottom_filler
• nocache_widget_render
• nocache_widget_render_instance
• normalize_align
• normalize_height
• normalize_valign
• normalize_width
• simplify_align
• simplify_height
• simplify_valign
• simplify_width

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)

Fix: #491

[coveralls]

Pull Request Test Coverage Report for Build 6341242230

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 74.38%

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Totals
Change from base Build 6340771828:	0.002%
Covered Lines:	7514
Relevant Lines:	10286

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💛 - Coveralls