mu4e: support showing 'baseline' query-results

Add the concept of a query results baseline, i.e., the result for
bookmark-queries at some particular point in time. Later, we can compare
the results with the then-current query-results.

Show the delta in the main view. Add mu4e-reset-baseline-query-results
to reset the baseline to 'now'. By default, we automatically reset when
explicitly (interactively) going to the main-view, i.e., M-x mu4e.

mu4e/mu4e-server.el

@@ -286,7 +286,7 @@ The server output is as follows:
 
          ;; received a pong message
          ((plist-get sexp :pong)
-	  (setq mu4e--server-props (plist-get sexp :props))
+          (mu4e--update-server-props (plist-get sexp :props))
           (funcall mu4e-pong-func sexp))
 
          ;; received a contacts message