Modify field selection when creating an export field mapping to mirror the approach used when adding fields to profiles

When selecting fields to be included in an export, the user is asked to select fields from various categories, including contact types, and others based on the type of export.

In the contact type categories there is no general 'contacts' option containing all the fields common to all contact types, as there is when one is selecting fields to be included in a profile.

This can result in needing to create more export fields than is necessary. A typical example will be when exporting contribution records: We need to include the display name (for example) as an export field, but the records we want to export range across multiple contact types. As a consequence of not being able to select "Contacts: display name" as a single export field, we need to create multiple export fields, one for each contact type we want to include in our export. For user organisations with many contact types, this could be be extensive.