XMPP attention branch

[Marcus Lundblad]

Speaking about the IBB file transfer branch, I came to think about the
xmpp.attention branch. This also consists of changes contained in the
XMPP prpl and also is updates the support to follow the XEP, which is
now status "draft". AFAIK this works as it should, now it also
advertizes support correctly (the code to send and retrieve attentions
using the old namespace of the XEP has been there since libpurple 2.2.x
sometime, AFAIK, but it wasn't advertizing support correctly).

Unfortunatly libpurple AFAIK is the only one yet supporting it :) Spark
has a similar feature called "poke", but it uses a custom extention.

Maybe this can soon be merged?

//Marcus