Fixed dialog not opening on consecutive page dropping

In sitemap, the dialog did not open up again if page is copied and then copied again.

To reproduce (without this pull):
- Go to sitemap
- Drag a page on top of another parent
- Select "copy"
- Try to drag that page again over the same parent
=> Dialog does not open up again

This is also mentioned here:
http://www.concrete5.org/developers/bugs/5-6-3-1/sitemap-scrambles-page-order-after-10-pages-copied/