folder
char const * folder_get_name (MailerFolder *folder);
MailerFolder *folder
FolderType folder_get_type (MailerFolder *folder);
void folder_set_type (MailerFolder *folder, FolderType type);
FolderType type
typedef struct _MailerFolder Folder;
FT_INBOX
FT_DRAFTS
FT_SENT
FT_TRASH
FT_FOLDER
# define FT_LAST FT_FOLDER
# define FT_COUNT (FT_LAST + 1)
typedef struct _AccountFolder AccountFolder;