weakref to enable gc + setting children with iterable + fix various tooltips

[fleming79]

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Replaces https://github.com/jupyter-widgets/ipywidgets/pull/3893. Commits have been tidied up.

Resolves

• [x] #3860
• [x] #3703
• [x] #728
• [x] #3440
• [x] #1345

[github-actions[bot]]

:point_left: Launch a binder notebook on branch fleming79/ipywidgets/weakref

[fleming79]

@vijayi1 Thanks for pointing that out. Attached are files for the latest .

files.zip

[vijayi1]

@fleming79 , the fix works for me, thanks. the table below summarizes the behavior with and without the fix for the text, dropdown, checkbox and button widgets. (note line 3, the behavior was inconsistent across widgets previously. if someone expects nothing to be displayed, with the fix they would have to set the tooltip to an empty string)