* src/client/conversation-viewer/conversation-viewer.vala: Convert to a
GtkStack. Use a GTK template for constructing the UI. Remove WebView
and any DOM-related code. Replace the enum DisplayMode and hence the
HTML spinner and HTML user message with widgets in the stack. Remove
all menus since they're all message specific and will need to be
re-implemented for ConversationViewer. Comment out composer related
code for the moment.
* src/client/application/geary-controller.vala
(GearyController::conversations_selected): Make both conversations and
current_folder arguments non-nullable, since it doesn't make any sense
for there not to be any and simplifies handler impls.
(GearyController::on_conversations_selected): Don't fire when there
isn't a current folder.
* src/client/components/main-window.vala (MainWindow::set_styling): Add
CSS theme code for the the conversation message list.
(MainWindow::create_layout): Remove GtkFrame, just add the
ConversationViewer instance directly.
* ui/conversation-viewer.ui: New GtkBuilder template for
ConversationViewer, implemented as a GtkStack containing widgets for
displaying the loading spinner, conversation as a GtkListBox, and label
for user messages.
* ui/CMakeLists.txt, po/POTFILES.in: Added new UI files.
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