set_icon
Description:
Sets the icon of this.
[class@TabBar] and [class@TabOverview] display the icon next to the title, unless [property@TabPage:loading] is set to `TRUE`.
`AdwTabBar` also won't show the icon if the page is pinned and [propertyTabPage:indicator-icon] is set.
Parameters:
| this | 
           a tab page  | 
      
| icon | 
           the icon of this  |