Moved HTMLRendererIMAPDataCallback to its own file.
* Added implementation.
* Updated include.
Assigning error in IMAP session rendering methods.
* Fixed method signature (ErrorCode is not a pointer).
Revert "Added simple rendering methods for IMAP message at C++ API level."

This reverts commit f04a1b069a075a18c6dbaf5a06fd1f8e4067d23c.