* src/client/conversation-viewer/conversation-message.vala: Add template
child widgets for displayed attachment UI, add signal for when user
activates an attachment.
(ConversationMessage::ConversationMessage): Ensure the attachment UI
is visible when there are displayed attachments.
(ConversationMessage::start_loading): New method for async loading of
message content, use just for attachments for now.
(ConversationMessage::on_attachments_view_activated): Handle activation
signal for specific attachments.
(ConversationMessage::load_attachments): Load attachments from the
message into the new UI.
(ConversationMessage::load_attachment_icon): Load icons for the
attachments UI from the attachment itself if an image, else from the
icon theme.
* src/client/application/geary-controller.vala: Hook up
attachment_activated signal to conversation messages.
* src/client/conversation-viewer/conversation-viewer.vala
(ConversationViewer::add_message): Ensure message loading starts after
new conversation messages are added to the window hierarchy.
* ui/conversation-message.ui: Add an attachments box, with an icon view
for displayed attachments.
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