activity import: better labels for custom fields
Previously, in the activity import, the dropdown that allowed you to select the field that you want to map a csv column to did not include the custom group title when listing custom fields. This lead to a usability issue in data models where custom field titles are repeated across groups and there is no way of distinguishing these fields in the import UI.
With this patch, custom field group titles are prepended to field title like this:
Custom group title :: custom field title.
This has the nice side effect that custom fields are grouped by custom group.
Also, it brings it inline with the behaviour of the contact import (win!)
See also https://github.com/eileenmcnaughton/nz.co.fuzion.csvimport/issues/37 for a similar issue fixed in the @eileen's csv api import extension.