Feature: Support FIXED-only widgets and FIXED rendering for PILE

[penguinolog]

• PileError subclass WidgetError
• Unify widget pack/render/input/cursor operation arguments collection
• LineBox support FIXED widgets and FIXED rendering due to implementation

Note: FIXED operations are not using caching for calculation

Tests for Pile moved to the separate file test_pile from test_container

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 7474063234

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 69.681%

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Totals
Change from base Build 7474041165:	0.3%
Covered Lines:	8203
Relevant Lines:	11856

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💛 - Coveralls

[wardi]

This looks great, I haven't gone over all the new tests but I assume the interface is somewhat consistent with the way Columns supports fixed widgets