Allow end users to move custom data fields to different groups

Implement support for moving a custom data field from one group to another.

There is a partial implementation here:

https://fisheye2.atlassian.com/browse/CiviCRM/branches/v3.4/CRM/Custom/Form/MoveField.php

And a few requirements (which may differ from the partial implementation):

1. The data-manipulation logic should go in CRM_Core_BAO_CustomField::moveField. The CRM_Custom_Form_MoveField::postProcess would call this helper.

2. The custom field ID should be preserved. This ensures that any mail-merge tokens, smart-groups, API consumers, etc. would continue to work.

3. The moveField() method should call a hook, passing along the old group ID, the new group ID, and the field ID.

4. The change should be backportable to 2.2. I don't think this would be challenging – but it's worth noting.