Moving away from positional addressing. Greater emphasis on EmailIdentifiers for

addressing.

Positional addressing is a nightmare for a lot of reasons, especially keeping positions
up-to-date as the Folder mutates.  Now, positions are returned with the email but for
advisory reasons only, and do not keep up-to-date.  It is expected that a client will use
positional addressing to "bootstrap" the application's data store, then use EmailIdentifier
addressing to traverse the Folder's contents.

src/client/ui/main-window.vala

@@ -200,7 +200,7 @@ public class MainWindow : Gtk.Window {
         current_conversations.lazy_load(-1, -1, Geary.Folder.ListFlags.FAST, cancellable_folder);
     }
     
-    public void on_scan_started(int low, int count) {
+    public void on_scan_started() {
         debug("on scan started");
     }