Implement MultiSelect

[willmcgugan]

We need a widget to select multiple options from a list.

Similar to HTML Select with multiple, but greater usability.

I think this should be built on OptionList, but have a checkbox by each item. The return value should be a tuple of values.

I don't think we need a overlay (like Select). Since this is designed to show multiple things, it probably should be multiple lines long, scrolling as required.

[Alphix]

I have a widget like that...but it's based on RadioSet rather than OptionList (and uses a bunch of CheckBoxes instead of RadioButtons...)

Do you want me to submit at PR?

The widget looks something like this:

[willmcgugan]

That looks very close to what we need! Could you share the code?

[Alphix]

@willmcgugan : sure, it's in PR #2524

[davep]

(In anticipation of the PR coming in tomorrow, and so I don't forget...) See also #2379.

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