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.el

@@ -139,6 +139,8 @@ invoke
   (unless (mu4e-context-current)
     (mu4e--context-autoswitch nil mu4e-context-policy))
   (setq mu4e-pong-func (lambda (info) (mu4e--pong-handler info func)))
+  (when mu4e-main-auto-reset-baseline
+    (mu4e-reset-baseline-query-results))
   (mu4e--server-ping
    (mapcar ;; send it a list of queries we'd like to see read/unread info for
     (lambda (bm)