[Pidgin] #7670: Implement xep-0191 (simple blocking) for jabber protocols

[Pidgin]

#7670: Implement xep-0191 (simple blocking) for jabber protocols
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 Reporter:  lukatmyshu                |     Owner:  deryni
     Type:  patch                     |    Status:  new   
Component:  XMPP                      |   Version:  2.5.2 
 Keywords:  jabber blocking xep 0191  |  
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 There is no support for blocking in jabber protocols (its only implemented
 for google).  Implement XEP-0191
 (http://xmpp.org/extensions/xep-0191.html).
 If the server capabilities contain GOOGLE, it reverts to the google-
 specific behaviour.

 With this patch, if you try to block someone and the server doesn't
 support blocking, we use purple_notify_error to signal an error.  Ideally
 the UI wouldn't let you block a user if your jabber server doesn't support
 blocking.  Also, the existing code doesn't signal any error if blocking
 isn't support (e.g. you're not using a google capabilities server).

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Ticket URL: <http://developer.pidgin.im/ticket/7670>
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