[Pidgin] #4509: Support XMPP Invisibility

[Pidgin]

#4509: Support XMPP Invisibility
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 Reporter:  js      |        Owner:  deryni
     Type:  defect  |       Status:  new   
Milestone:          |    Component:  XMPP  
  Version:  2.3.1   |   Resolution:        
 Keywords:          |  
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Comment(by sundaymorning):

 It seems that gtalk invisibility discussion should be done here. That's my
 interpretation after #9392 have been marked as a duplicate of this bug.

 I present you with a less-than-perfect patch. It has been working fine for
 my needs and it could benefit others, that's why I'm uploading it. I don't
 think it should be integrated into the mainstream code because it's hacky
 and doesn't give the user "strong" invisibility. Feedback and suggestions
 are welcome, though.

 My patch allows the user to be invisible while still being able to see who
 logs in and out, send messages and receive messages while invisible. The
 invisibility is not strong in the sense that someone who's really
 interested into knowing if you're online is able to find that out.

 Google's implementation of invisibility also allows for someone who really
 wants to find out if you are invisible to do so. Therefore, I think my
 patch is no worse than google implementation in that sense.

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Ticket URL: <http://developer.pidgin.im/ticket/4509#comment:23>
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