MultiSelect & MultiCheckBox fixes

[yha]

This PR:

• Fixes #218 for both MultiSelect and MultiCheckBox
• Shares code between the two widgets, so now the fix of #106 also applies to MultiSelect
• Fixes a bug where the widget would return the default array as is, so it could change type when making the first selection. Now the widgets always return Vector{BT} values.
• Adds more thorough testing, and subsequent fixes to Bonds methods which didn't work at all

I moved the MultiSelect implementation to the MultiCheckBox.jl notebook to share code between them, but I'm not sure why MultiCheckBox is in a separate notebook in the first place. May it should be renamed or merged into Builtins.jl?

[github-actions[bot]]

Try this Pull Request!

Open Julia and type:

  julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/yha/PlutoUI.jl", rev="multiselect-fixes")
julia> using PlutoUI

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