endSessionDialog: clean up appearance

This commit lines up the app list with the description,
adds spacing between controls of the dialog, and increases
the wrap width of the text.

https://bugzilla.gnome.org/show_bug.cgi?id=641375

js/ui/endSessionDialog.js

@@ -233,7 +233,7 @@ EndSessionDialog.prototype = {
     __proto__: ModalDialog.ModalDialog.prototype,
 
     _init: function() {
-        ModalDialog.ModalDialog.prototype._init.call(this);
+        ModalDialog.ModalDialog.prototype._init.call(this, { styleClass: 'end-session-dialog' });
 
         this._user = Gdm.UserManager.ref_default().get_user(GLib.get_user_name());