Expand C API for Configuration

[devunwired]

Feature Request: Include basic access to configuration keys from the C API, including init, load/save, declare/get configurations.

The Configuration interface has the beginnings of a C API (Configuration_c.h) focused primarily around adding a custom configuration container, but it is currently missing most of the core functions from Configuration.h

A good starting point would be to allow the existing C-API unit tests to run entirely using C API functions without help from the C++ interface.