fix(selection-list): tabIndex should respect disabled state

[devversion]

• Currently if the selection-list is disabled, the tabIndex may be still set to a valid value that allows tabbing to the element. The mixinTabIndex respects the disabled state of the selection list.

[crisbeto]

Code-wise LGTM, but I'm not sure whether disabling focus on the list is the way to go since it's also a presentational component. Thoughts @jelbourn?

[devversion]

@crisbeto Not too sure about that. The selection list seems more intended to be an interactive control than for presentation. But I see what you mean. Maybe in that case it would make more sense to use a normal list.

[jelbourn]

LGTM, selection-list is specifically a form contorl (listbox), so this should be the right behavior

[andrewseguin]

Needs rebase.

[kara]

@devversion Rebase one more time?

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