Slider: Suppress the click events of disabled handles

[tjvantoll]

Fixes #9890

[scottgonzalez]

I'd prefer to look into http://bugs.jqueryui.com/ticket/7163 before making this change.

[jcarlson]

Good grief. That PR came in faster than I could at least submit my failing spec to illustrate the issue! Thanks!

[jcarlson]

Re: http://bugs.jqueryui.com/ticket/7163, I'd love to see the removal of 'href' attributes from things that don't have URLs... the '#' values are not very semantically useful. In the meantime, this PR resolves the regression without preventing a change to tabindex="0" in the future, so I would vote to move forward with this change.

[tjvantoll]

Well really, there's no reason the handles should even be <a> elements that I can think of. Handles using <span tabindex="0"> elements seem to work just fine in IE 8 - 11, Firefox, Chrome, and Safari. I'll do more thorough testing tomorrow, but I think we can just switch to <span>s and avoid this problem altogether.

[tjvantoll]

Alternative PR: #1207.

[scottgonzalez]

Let's go with the alternative.