Editor: Resolution of several issues in IE 11.

IE 11 now supports [`Node.textContent`](https://developer.mozilla.org/en-US/docs/Web/API/Node.textContent?redirectlocale=en-US&redirectslug=DOM%2FNode.textContent), and [`Window.getSelection`](https://developer.mozilla.org/en-US/docs/Web/API/window.getSelection?redirectlocale=en-US&redirectslug=DOM%2Fwindow.getSelection).

Unfortunately, Y.Editor relies heavily on user agent strings, and therefore a couple of places needed updating.