edit text cells on double-click instead of single-click

Single-click to edit gets in the way of using interactive elements (e.g. non-flash videos),
and select/copy of the rendered HTML.  Switching to double-click makes the edit action more intentional.

IPython/frontend/html/notebook/static/js/textcell.js

@@ -89,7 +89,7 b' var IPython = (function (IPython) {'
     TextCell.prototype.config_mathjax = function () {
         var text_cell = this.element;
         var that = this;
-        text_cell.click(function () {
+        text_cell.dblclick(function () {
             that.edit();
         }).focusout(function () {
             that.render();