Automatically set identifier to "Preferred" when adding a new patient

When adding a new patient, the initial identifier "Preferred" radio button should be selected by default. If it's not manually ticked (even when only one identifier is added), it will end up creating a patient with no preferred identifier. Having the first identifier set as preferred by default would save a click when adding new patients, and ensure more conformed data if the users forgets to manually check it.

This behavior is exhibited in OpenMRS Version: 1.6.0 Build 12644, and on the web demo Version: 1.6.0 Build 12685.