iconGrid: Don't recreate StIcons on icon theme changes

Now that StIcon updates its texture automatically on icon theme
changes, we only have to recreate icon actors that aren't StIcons.

(This probably only applies to the folder icon in the app grid
where the sub-icons do use St.Icon, but making that assumption
feels dodgy with an API as generic as `createIcon()`)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3141>

js/ui/iconGrid.js

@@ -156,6 +156,8 @@ class BaseIcon extends Shell.SquareBin {
     }
 
     _onIconThemeChanged() {
+        // St.Icon updates automatically
+        if (!(this.icon instanceof St.Icon))
             this._createIconTexture(this.iconSize);
     }