Requires GTK+ 3.12.

* src/client/conversation-viewer/conversation-message.vala
  (ConversationMessage::link_selected): Added to pass through successful
  link clicks to the message_viewer, hook it up to the web_view.
  (ConversationMessage::load_message_body): Hook up WebKit event handler
  for when links are clicked, so phishing links can be intercepted.
  (ConversationMessage::on_link_clicked_self): Use a popover to display
  phishing warning, recursively check link's offset parent's when
  calculating box position, escape link text/href before using it as
  markup.

* src/client/conversation-viewer/conversation-viewer.vala
  (ConversationViewer::add_message): Ensure unhandled mouse clicks o the
  web_view are not used to activate the message's ListBoxRow.

* ui/conversation-message.ui: Add popover (GTK+ 3.12) for phishing links.