folderView: Move DnD functions to BaseAppView

It will be much easier to handle Drag n' Drop with BaseAppView
implementing default handlers for it.

Move handleDragOver() and acceptDrop() to BaseAppView.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/603

js/ui/appDisplay.js

@@ -192,6 +192,14 @@ class BaseAppView {
             log(`No such application ${id}`);
     }
 
+    handleDragOver(source, actor, x, y, time) {
+        return DND.DragMotionResult.NO_DROP;
+    }
+
+    acceptDrop(source, actor, x, y, time) {
+        return false;
+    }
+
     selectApp(id) {
         if (this._items[id] && this._items[id].actor.mapped) {
             this._selectAppInternal(id);