prevent esc from bubbling up when dismissing tooltip

prevents esc from entering command mode when it's
meant to dismiss the tooltip.

The logic for the event was already there,
it just lacked the `ipkmIgnore` bit.

IPython/html/static/notebook/js/codecell.js

@@ -329,6 +329,7 @@ define([
             // If we closed the tooltip, don't let CM or the global handlers
             // handle this event.
             event.codemirrorIgnore = true;
+            event._ipkmIgnore = true;
             event.preventDefault();
             return true;
         } else if (event.keyCode === keycodes.tab && event.type === 'keydown' && event.shiftKey) {