dash: Restore icon's expected DND behavior

Commit ff3d32dd18 added a custom DashIcon subclass that disables
all DND methods from ancestors, including canceling the context
menu timeout and emitting the overview's item-drag-begin signal.

All we want is opting out of the parent's scale-and-fade behavior,
so override those methods instead.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3209

js/ui/dash.js

@@ -31,11 +31,11 @@ class DashIcon extends AppDisplay.AppIcon {
         });
     }
 
-    // Disable all DnD methods
-    _onDragBegin() {
+    // Disable scale-n-fade methods used during DND by parent
+    scaleAndFade() {
     }
 
-    _onDragEnd() {
+    undoScaleAndFade() {
     }
 
     handleDragOver() {