* src/client/application/geary-controller.vala (GearyController): Remove
  ACTION_FIND_NEXT_IN_CONVERSATION and
  ACTION_FIND_PREVIOUS_IN_CONVERSATION arctions and callbacks since they
  will be taken care of by the search entry & search bar buttons, and
  remove from accelerators.ui. Add ACTION_TOGGLE_FIND action to handle
  toggling find bar in the same way as the search bar.

* src/client/components/main-toolbar.vala (MainToolbar): Add new button
  and infrastrcuture for toggling the find bar.

* src/client/conversation-viewer/conversation-viewer.vala
  (ConversationViewer): Convert ::conversation_page to be grid, add new
  ::conversation_scroller property for the scrollbar, update call
  sites. Add props for accessing find widgets, remove old find methods
  and add callbacks for handling find start, change, etc.

* src/client/conversation-viewer/conversation-email.vala,
  src/client/conversation-viewer/conversation-message.vala: Add methods
  for accessing selected text for find.

* src/client/conversation-viewer/conversation-listbox.vala
  (ConversationListBox::highlight_search_terms): Updated to return a flag
  specifiying whether any search results were found, and to
  expand/collapse messsages depending on whether they have any.

* src/client/conversation-viewer/conversation-message.vala
  (ConversationMessage::highlight_search_terms): Keep track of how many
  results were found, and return that.

* ui/conversation-viewer.ui: Convert conversation_page to be a grid, add
  a search bar and search widgets to it, and move conversation
  ScrolledWindow to it.