Added widget_addon to choice_widget_collapsed

Added widget_addon to the choice field in order to make content before or after the <select> field possible.
With this change for example a button after the field is possible.

![bildschirmfoto 2013-08-27 um 10 02 40](https://f.cloud.github.com/assets/2010989/1031879/115d06b2-0eef-11e3-964c-cd9f69e02d7f.png)