Ofline message flag

[Jorge Villaseñor]

I don't know about other protocols but MSNP14 allow offline messages.
the problem here is that when received they take the same color coding
than a _normal_ im and if the person that sent the offline message is
connected and sends you a new (online) message you will think that all
messages (the offline and online) are offline ones.

So I want to patch the gtkconv to use a different nick color for
offline message. I'm thinking in gray instead of red.

The problem here is that gtkconv can't define if it is a normal
message or a offline one. I need to add a PURPLE_MESSAGE_OFFLINE flag
to the PurpleMessageFlags enum. What do you think about this ?

At this moment the last flag =0x8000 so the new one should be 0x10000

I can also set the message flag to PURPLE_MESSAGE_SYSTEM I just tested
this one, I got the message in Black. but I think thats unnecesary
hacky.