item_adds bug fix

Right now, the dashboard groups config parameter 'item_adds' is not working at all , as documented here: https://github.com/sonata-project/SonataAdminBundle/pull/668

However, the above mentioned fix ignores the 'items' config option if the 'item_adds' exists, and just adds the specified admin mappings on top of the default one.

Tests for this can be found here: https://github.com/sonata-project/SonataAdminBundle/pull/1405