[Pidgin] #1324: Add signals for email notification

[Pidgin]

#1324: Add signals for email notification
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 Reporter:  charkins      |       Owner:       
     Type:  patch         |      Status:  new  
 Priority:  minor         |   Milestone:  2.1.0
Component:  pidgin (gtk)  |     Version:  2.0.1
 Keywords:                |     Pending:  0    
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 This patch adds signals corresponding to purple_notify_email() and
 purple_notify_emails(). I've also added appropriate callbacks in the
 signals-test plugin (though I stop short after displaying 5 emails in
 response to displaying-emails-notification signal). I have limited
 experience using email notification, so this could use a little testing
 from others.

 These signals will be necessary to implement sound events for mail
 notification (ticket #780) and could help in dealing with tickets #530 and
 #531.

 As this adds new API (signals count as API, right?), I'm setting milestone
 to 2.1.0.

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Ticket URL: <http://developer.pidgin.im/ticket/1324>
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