Add option to allow to use 'Wrap' widget for FormBuilderCheckboxList

[magillus]

One of my use cases is that the form have multi selection on wide area.
With using current implementation there is a lot of space between label and checkbox, using Wrap widget and forcing child (checkbox with label) widget to have set width would make it look like expandable table of check boxes.

[danvick]

Hello @magillus,
I'm aware of the space issue and I'm working on fixing that without breaking changes, I'm be making such improvements to the next major version of the package.

In the meantime, I'd advise setting leadingInput to true so that the Checkbox is next to the label.

[magillus]

Thank you for great package.
If I had little more spare time I would contribute PR, for now just using local change of Wrap and static width of checkboxes.

thx for tip on leadingInput