[Tabs] Use MutationObserver to update tab header pagination

[andrewseguin]

The tab header displays scrolling that updates when the header is initialized or when tabs are removed/added. This misses the case where the tab label's text changes and the scrolling should reflect the update.

Add a MutationObserver that notifies changes to the text node and lets the scrolling react to the change.

[jelbourn]

@EladBezalel could you take a pass at this? I want to create a general directive in core that just fires an event when the text content of an element changes uses a MutationObserver. The tabs should then use that as a signal to recalculate the width of the tab bar (for scrolling)

[EladBezalel]

closed with #2186

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