Change z-index of ipython_tooltip

I experienced the problem stated in  #5755. Giving the ipython_tooltip class a higher z-index indeed solves the problem. I intentionally set it to a high value so it likelier remains on top when styles are changed in the future.

IPython/html/static/notebook/less/tooltip.less

@@ -117,7 +117,7
     };
     position: absolute;
 
-	z-index: 2;
+	z-index: 1000;
 	
 	.tooltiptext {
 		pre {