Refactoring: move listbox & treetools modules to the widgets

[penguinolog]

• all main widgets located together
• Base package imports nested packages last
• Remove sorting magic from base package: no import hell anymore

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 8005035902

Details

• 0 of 37 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 72.524%

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Totals
Change from base Build 8003945851:	0.02%
Covered Lines:	9148
Relevant Lines:	12677

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💛 - Coveralls