st/icon: Automatically update on icon theme changes

St.Icon is very commonly used for displaying themed icons, yet so far updating the icon on icon theme changes is left to the caller.

Unsurprisingly, very few actually do that, with the result that for most icons, icon theme changes only take effect after a delay (say, a color change on hover) or not at all.

This is also inconsistent with GTK, where Gtk.Image will automatically pick up icon theme changes.

Address this by tracking whether the current icon corresponds to a themed icon, and update it automatically on theme changes if it does.