Generate documentation for libemail-engine.
Rename e-mail-enums.h to e-mail-engine-enums.h.
Fix misspelled enum type.
Remove unused EMailDisplayStyle enum.
Generate enum GTypes for libevolution-mail.

Steals some enum types that libemail-engine doesn't need to know about.
Move EMailImageLoadingPolicy to e-mail-formatter-enums.h.

libemail-engine doesn't need to know about this enum.
Convert libemail-engine to a single-include model.

Use: #include <libemail-engine/libemail-engine.h>