Make Show "Slug" value available for mapping.

Problem/Motivation

This value is available in the source but cannot be mapped because the code limits to some specific values only.

Steps to reproduce

- Go to /admin/config/services/media-manager/show-mappings
- You cannot configure the mapping Value -> Field for Show Slug

Proposed resolution

I created a single patch that allowed me to make it possible, this is simple but I think the whole function can be refactored to handle even more values that are available.

Remaining tasks

Review the patch.
Consider refactoring the code to allow more field/value mapping.