Fix enabled packages reset when changing target platform

Steps to reproduce:

• select the DM package on CAP 5.7, then save
• change the target plaftorm to CAP 5.6, do not select any packages, and save

=> Logs show errors like:

2013-05-29 09:59:30,579 ERROR [com.nuxeo.studio.core.platforms.TargetPlatform] Package 'nuxeo-dm-5.7' is not available for target platform 'cap-5.6'

So the target package nuxeo-dm-5.7 is not correctly "unselected" when changing the target platform.