MatTab: selectedTabChange should not fire when the tab is changed programmatically

There should be a way to differentiate **between a UI and a programmaticcally induced tab change**. As of now, both cause the selectedTabChange: EventEmitter to emit.

My case is pretty simple: I want to display the currently opened Tab in the URL:

somePage?openedTab=1
somePage?openedTab=2

But I also want the corresponding tab to open when a URL pointing to a specific tab is navigated to. **Ideally, the [selectedIndex] input should not cause (selectedTabChange) to fire**.

Does this make sense?

Material 8.2.3, Angular 8.2.11

从 issue 中描述的 MatTab 的 selectedIndex input 和 selectedTabChange EventEmitter 行为入手。确定如何区分 UI 变更和程序变更，然后验证由 URL 驱动的变更不会触发 UI 变更通知，而用户进行的标签页变更仍会触发该通知。

由索引模型根据 Issue 内容生成。