Only set .onboarding-focus class when onboarding is enabled

[kogre]

Whenever the onboarding is not (yet) active, the onboarding-focus class is set on the element of the first step. This might conflict with some existing z-index attributes, yielding unwanted behavior (as it did in my case).

This PR runs the update logic on changes to index as well as enabled, and only sets the onboarding-focus class on elements when the onboarding is active.

[atulherenow]

@adamalbrecht

Attached item is not focused in case of <a> tags sometimes even in <div>

Sorry to ask this here as issues tab is disabled. . . . . Thanks for the awesome plugin.