Display field destinations sub-properties

Problem/Motivation

When defining a field mapping you'll need to have the ability to separate multi field properties to be imported from different source fields. For example, if you have a field mapping of body and then mapped that to some content. Next you added another field mapping to account for the body formatter, like so body/format. When that import is processed it would overwrite the body definition with the body/format array instead.

Proposed Solution

The way around that would be to use field destination of body/value to map the content. This way the content won't get overwritten. We'll need to update the field mapping destination options to account for this change. Everything will remain BC.