[prototype->jQuery] Removed prototype usage of observeProjectModules()

This refactoring helps to remove remaining prototype code.

This can be tested in the create project form (administration -> project -> new project) when toggling the module "work package tracking". The" Types" fieldset should toggle its visibility.

![selection_032](https://cloud.githubusercontent.com/assets/206108/2829532/1deb2e42-cfa4-11e3-9a14-f3d1af7423ba.png)

see:https://www.openproject.org/work_packages/7066