indicator_icon
Description:
An indicator icon for the page.
A common use case is an audio or camera indicator in a web browser.
[class@TabPage] will show it at the beginning of the tab, alongside icon representing [property@TabPage:icon] or loading spinner.
If the page is pinned, the indicator will be shown instead of icon or spinner.
If [property@TabPage:indicator-activatable] is set to `TRUE`, the indicator icon can act as a button.