* src/folder.[ch]

* src/folder.[ch]
* src/msgcache.[ch]
        Added message-id table to the cache in memory
        and functions to msgcache.c and folder.c to
        fetch MsgInfos by message id
* src/compose.c
* src/procmsg.c
        store folder and message-id of message replying
        to in queue header and set reply flag after sending
        (Closes bug #469498)