Display attached+un-embedded composer as an additional ConversationViewer
stack page, although it realy should be broken as its own top-level
widget - there's already too much state in ConversationViewer.

* src/client/conversation-viewer/conversation-viewer.vala: Remove old
  composer boxes code. Add new ViewState enum, property and methods to
  define and manipulate the current view state - either conversation or
  composer.
  (do_conversation): New method to put the viewer in conversation mode.
  (do_compose): New method to put the viewer in compose mode, hook up the
  composer widget, and handle ConversationListView selection management
  for now.
  (on_folder_selected, on_conversation_count_changed,
  on_conversations_selected): Ensure these methods do the right thing
  depending on the viewer's current view state.
  (set_paned_composer): Replaced by ::do_compose, fixed call sites.
  (show_multiple_selected): Minor code clean up - moved down to a more
  appropriate location.

* src/client/composer/composer-box.vala (ComposerBox): Don't attempt to
  up-manage it's parent's state, since the parent has a much better idea
  of how best to do that. Likewise move code to manage previous
  ConversationList selection out, provide signal so the a more
  appropriate class can manage it instead.

* src/client/composer/composer-container.vala: Add some method comments.

* ui/conversation-viewer.ui: Add new page to the stack for the composer.