[Pidgin] #16678: Add initiate_chat to protocol buddy methods

[Pidgin]

#16678: Add initiate_chat to protocol buddy methods
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 Reporter:  EionRobb     |      Owner:
     Type:  enhancement  |     Status:  new
Milestone:  3.0.0        |  Component:  libpurple
  Version:  3.0.0hg      |   Keywords:
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 Various prpls have over the years had custom blist_node_menu's to be able
 to add an 'Initiate Chat' option to start a multi-user chat room with that
 buddy as the initially invited user - see MSN or XMPP (Google) for
 examples that first come to mind.

 Client UI's such as Adium have added hacks to wrap the Initiate Chat
 options into a nice UI option, but from a Purely Pidgin POV this could be
 added as an 'Initiate Chat with...' option in the Buddies menu in the
 buddy list.

 Please include the ability for a prpl to map a method/function for
 allowing the UI to hook into this functionality in a consistent way.

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Ticket URL: <https://developer.pidgin.im/ticket/16678>
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