Honor tabindex if set, save it when disabling a component.

[depeele]

Add _updateAccessabilityAttributes() to components that use tabindex for the purpose of managing accessability attributes, including tabindex:

• honor any existing tabindex attribute on the source component;
• if there is no tabindex, initialize it to '0';
• when a component is disabled, save the original tabindex value in the data-tabindex attribute and set tabindex to '-1';
• when a component becomes enabled, if there is a data-tabindex attribute, restore the tabindex value stored there and remove the data-tabindex attribute;

[jarek-foksa]

Is there any reason why you are storing the previous tabindex as an attribute:

this.setAttribute('data-tabindex', tabIndex);

... rather than as a JS object property (where $tabIndex is a Symbol):

this[$tabIndex] = tabIndex;?

The only drawback of the latter approach I can think of is that the old tabindex might get lost when using virtual DOM.

[depeele]

The only reason was to try and keep attributes associated with DOM elements. If the DOM tree is cloned for any reason, the data-tabindex goes along automatically.

It could work just as well as a property of the instance, using a $tabIndex symbol...

I just ran across the need to honor the tabindex that was set on the element and approached it this way...

The "right" approach is really your call.

[jarek-foksa]

I have refactored your code to use symbols. Let me know in case you notice any issues.