Clean up attachment save code

Create new Application.AttachmentManager class and move code for saving
attachments there from both Application.Controller and MainWindow since
aside from needing the latter for dialogs it is independent of both.

Create new Components.AttachmentPane widget for diplaying attachents
for an email in the ConversationViewer.

Update ConversationEmail and ConversationMessage to use these two new
classes directly, rather than implementing save management itself or
requiring the MainWindow to wire up signals on these classes.

ui/geary.css

@@ -198,6 +198,16 @@ grid.geary-message-summary {
   border-radius: 0px;
 }
 
+/* AttachmentPane  */
+
+.geary-attachment-pane flowbox {
+  margin: 6px;
+}
+
+.geary-attachment-pane flowboxchild {
+  border-radius: 3px;
+}
+
 /* PlaceholderPane  */
 
 .geary-placeholder-pane.geary-has-text > image {