Notification Attention per Vladimir Panteleev

When a page displays a notification, additionally "draws attention" to the containing window.

The exact effect of this extension is platform-specific. To quote the used (formerly-public) function's documentation:

> On Windows, the taskbar button for the window flashes, if this hasn't been disabled by the user.
>
> On Linux, the behaviour varies from window manager to window manager - some flash the taskbar button, others focus the window immediately. This may be configurable as well.
>
> On Macintosh, the icon in the upper right corner of the desktop flashes.