Allow using a Function as the cancel option instead of a Selector.

[kghandi]

This is particularly useful when using Sortable on nested lists.
I had a similar issue to that listed in issue #1996. Allowing cancel option to be a function allows me to override the default cancel implementation using a selector with something like:

$('.container', context).sortable({
items: '>.widget',
cancel: function (event) {
return $(event.target).closest('.widget').is('.locked');
}
});

[linux-foundation-easycla]

• ✅login: kghandi / (b100ed8)

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[mgol]

Thanks for the contribution. This change makes sense to me, although before formally accepting I'd like @fnagel to also voice his opinion for possible wider context.

If Felix agrees this makes sense, there are still a number of changes we need before accepting this PR:

1. Make sure you follow our style guide: https://contribute.jquery.org/style-guide/js/. Currently, indentation is wrong, it uses spaces instead of tabs, spaces are missing inside of parens, etc.
2. Connected to the above - make sure our tests & linters succeed on the code before asking for another review. In this case, ESLint fails, for example. Run npm install && npm run build && npm run lint && npm test locally and see if everything works.
3. This change needs tests in all affected modules: draggable, sortable, etc. - perhaps whatever depends on $.ui.mouse.
4. This needs API updates so we'll need a PR to https://github.com/jquery/api.jqueryui.com/ as well before merging any new API.

[fnagel]

Sounds reasonable to me and the changes are not that big, so I'm fine with integrating this when all requirements are met.