Fix mu4e-main revert-buffer-function and use it

* mu4e/mu4e-headers.el (mu4e~headers-quit-buffer): Refresh main buffer
when done.
* mu4e/mu4e-main.el (mu4e-main-mode-map): Don't bind "g" to mu4e, "g"
should be bound to revert-buffer (special-mode).
(mu4e-main-mode): No need to specify map.
(mu4e~main-view-real-1): New.
(mu4e~main-redraw-buffer): New.
(mu4e~main-view-real): Use them.
(mu4e~main-view): Take one more arg REFRESH.
(mu4e~main-toggle-mail-sending-mode): revert-buffer when done.

mu4e/mu4e-headers.el

@@ -1898,7 +1898,7 @@ other windows."
       ;; now, all *other* windows should be gone. kill ourselves, and return
       ;; to the main view
       (kill-buffer)
-      (mu4e~main-view))))
+      (mu4e~main-view 'refresh))))
 
 ;;; _
 (provide 'mu4e-headers)