PURPLE_MESSAGE_DELAYED flag

[Mark Doliner]

We have this handy flag that signals to the core that an incoming IM
or chat message is "delayed."  Our sound playing code doesn't play a
sound if the flag is set.  The Message Notification plugin doesn't
notify if the flag is set.  And the Finch GntHistory plugin sets the
flag when prepending recent conversation history into an IM window.

The ONLY place this flag is set is when receiving a "delayed" Jabber
conference message.  This has generally been used to send you the
recent room history when you join a conference room.

My question: Is there any reason we're not setting this flag all over
the place?  Like when receiving an offline IM on all our protocols?  I
feel like there was a reason we weren't doing this for IMs, but I
don't remember what it was.  And it might be nice for UIs to know that
a message is delayed without having to look at the timestamp on the
message.

-Mark