[FIX] Context settings

[thocevar]

Problem: Open a widget several times (e.g. Box plot) and change the selected attribute each time. It works only the first and second time. After that (third, fourth time) it will remember the second setting.

Fix:

• when matching a context, consider lists as valid items
• don't move up the matched global context if we'll add a new one anyway

[codecov-io]

Current coverage is 88.69% (diff: 100%)

Merging #1577 into master will not change coverage

@@             master      #1577   diff @@
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