The conversation viewer's ListBox is sufficiently complex to warrant its
own widget. Use empty placeholders for the list per the HIG, and
correctly fix mamagement of empty folder vs no conversations selected
this time.

* src/client/application/geary-controller.vala (GearyController):
  Directly manage secondary parts of the conversation viewer, since the
  controller since it has a better and more timely idea of when a
  conversation change is due to folder loading status or from the user
  selecting conversations, and so the viwer doesn't need to hook back
  into the controller. Remove the now-unused conversations_selected
  signal and its callers.

* src/client/conversation-viewer/conversation-listbox.vala: New widget
  for displaying the list of emails for a conversation. Moved relevant
  code from ConversationViewer here. Made adding emails async to get
  better UI responsiveness. Don't implement anything to handle
  conversation changes or emptying the list.

* src/client/conversation-viewer/conversation-viewer.vala: Replace user
  messages - empty folder/search & no/multiple messages selected with new
  EmptyPlaceholder. Remove a lot of the state manage code needed when
  managing the email listbox. Add a new ConversationListBox for every new
  conversation and just throw away.

* src/client/conversation-list/conversation-list-view.vala
  (ConversationListView): Clean up firing the conversations_selected
  signal - don't actually emit it when the model is clearing, and don't
  bother delaying the check either.

* src/client/components/empty-placeholder.vala: New widget for displaying
  empty list and grid placeholders per the HIG.

* src/client/conversation-viewer/conversation-email.vala
  (ConversationEmail): Make manually read a property, since it
  effectively is one.

* src/CMakeLists.txt: Include new source files.

* po/POTFILES.in: Include new source and UI files, and some missing ones.

* ui/CMakeLists.txt: Include new UI files.

* ui/conversation-viewer.ui: Replace user message and splash page with
  placeholders for the new empty placeholders(!).

* ui/empty-placeholder.ui: UI def for new widget class.

* ui/geary.css: Chase widget name/class changes, style new
  empty placeholder UI.