LibGUI: Allow dropping drags on AbstractView

You can now drop things on an AbstractView, which will ask its model if
the drag is acceptable to drop at the index where it's dropped.

If it's accepted by the model, the view will fire the on_drop hook.

Libraries/LibGUI/AbstractView.cpp

@@ -326,4 +326,19 @@ void AbstractView::context_menu_event(ContextMenuEvent& event)
         on_context_menu_request(index, event);
 }
 
+void AbstractView::drop_event(DropEvent& event)
+{
+    event.accept();
+
+    if (!model())
+        return;
+
+    auto index = index_at_event_position(event.position());
+    if (!index.is_valid())
+        return;
+
+    if (on_drop)
+        on_drop(index, event);
+}
+
 }