dash/dashIcon: Popup menu above the icon

Now that the Dash is horizontal, the popup menu of the Dash icons must
show up, instead of left/right.

Make AppIcon.popupMenu() receive an optional parameter with the side
to show the menu, using St.Side.LEFT as default. Override this method
in DashIcon to always pass St.Side.TOP.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1559>

js/ui/dash.js

@@ -31,6 +31,10 @@ class DashIcon extends AppDisplay.AppIcon {
         });
     }
 
+    popupMenu() {
+        super.popupMenu(St.Side.BOTTOM);
+    }
+
     // Disable scale-n-fade methods used during DND by parent
     scaleAndFade() {
     }