Fix thread 15450 - TableTools collection button can spawn multiple

backgrounds

- Due to a z-indexing error, the background element could actually be
  created multiple times by clicking on the element without moving the
  mouse. Adding a z-index statement fixes this.

integration/bootstrap/dataTables.bootstrap.css

@@ -110,6 +110,9 @@ div.DTTT .btn:hover {
 	text-decoration: none !important;
 }
 
+ul.DTTT_dropdown.dropdown-menu {
+  z-index: 2003;
+}
 
 ul.DTTT_dropdown.dropdown-menu a {
 	color: #333 !important; /* needed only when demo_page.css is included */