Respect switch-function in compose-mail.

Emacs has several standard keybindings

C-x m   compose-mail
C-x 4 m compose-mail-other-window
C-x 5 m compose-mail-other-frame

This patch fixes the creation of new mail buffers to respect the
latter two keybindings, C-x 4 m and C-x 5 m.

Note that there is already the variable mu4e-compose-in-new-frame
which if true opens in a new frame.  That will still work for C-x m
and C-x 5 m, but if the user runs C-x 4 m, it switches to other-window
as it assumes the keybinding takes precedence.  This behaviour can be
changed within mu4e~draft-open-file.

mu4e/mu4e-compose.el

@@ -676,7 +676,8 @@ See `mu4e-compose-crypto-policy' for more details."
           (sign (mml-secure-message-sign))
           (encrypt (mml-secure-message-encrypt)))))
 
-(cl-defun mu4e~compose-handler (compose-type &optional original-msg includes)
+(cl-defun mu4e~compose-handler (compose-type &optional original-msg includes
+                                             switch-function)
   "Create a new draft message, or open an existing one.
 
 COMPOSE-TYPE determines the kind of message to compose and is a
@@ -710,7 +711,7 @@ tempfile)."
   ;; this opens (or re-opens) a messages with all the basic headers set.
   (let ((winconf (current-window-configuration)))
     (condition-case nil
-        (mu4e-draft-open compose-type original-msg)
+        (mu4e-draft-open compose-type original-msg switch-function)
       (quit (set-window-configuration winconf)
             (mu4e-message "Operation aborted")
             (cl-return-from mu4e~compose-handler))))
@@ -971,7 +972,7 @@ draft message."
 
 ;;;###autoload
 (defun mu4e~compose-mail (&optional to subject other-headers _continue
-                                    _switch-function yank-action _send-actions _return-action)
+                                    switch-function yank-action _send-actions _return-action)
   "This is mu4e's implementation of `compose-mail'.
 Quoting its docstring:
 Start composing a mail message to send.
@@ -1009,7 +1010,7 @@ buffer buried."
 
   ;; create a new draft message 'resetting' (as below) is not actually needed in this case, but
   ;; let's prepare for the re-edit case as well
-  (mu4e~compose-handler 'new)
+  (mu4e~compose-handler 'new nil nil switch-function)
 
   (when (message-goto-to) ;; reset to-address, if needed
     (message-delete-line))

mu4e/mu4e-draft.el

@@ -523,11 +523,15 @@ will be the same as in the original."
   "The drafts-folder for this compose buffer.
 This is based on `mu4e-drafts-folder', which is evaluated once.")
 
-(defun mu4e~draft-open-file (path)
+(defun mu4e~draft-open-file (path switch-function)
   "Open the the draft file at PATH."
-  (if mu4e-compose-in-new-frame
-      (find-file-other-frame path)
-    (find-file path)))
+  (let ((buf (find-file-noselect path)))
+    (funcall (or 
+              switch-function
+              (and mu4e-compose-in-new-frame 'switch-to-buffer-other-frame)
+              'switch-to-buffer)
+             buf)))
+
 
 (defun mu4e~draft-determine-path (draft-dir)
   "Determines the path for a new draft file in DRAFT-DIR."
@@ -535,7 +539,7 @@ This is based on `mu4e-drafts-folder', which is evaluated once.")
           (mu4e-root-maildir) draft-dir (mu4e~draft-message-filename-construct "DS")))
 
 
-(defun mu4e-draft-open (compose-type &optional msg)
+(defun mu4e-draft-open (compose-type &optional msg switch-function)
   "Open a draft file for a message MSG.
 In case of a new message (when COMPOSE-TYPE is `reply', `forward'
  or `new'), open an existing draft (when COMPOSE-TYPE is `edit'),
@@ -555,7 +559,7 @@ will be created from either `mu4e~draft-reply-construct', or
        ;; full path, but we cannot really know 'drafts folder'... we make a
        ;; guess
        (setq draft-dir (mu4e~guess-maildir (mu4e-message-field msg :path)))
-       (mu4e~draft-open-file (mu4e-message-field msg :path)))
+       (mu4e~draft-open-file (mu4e-message-field msg :path) switch-function))
 
       (resend
        ;; case-2: copy some exisisting message to a draft message, then edit
@@ -563,7 +567,7 @@ will be created from either `mu4e~draft-reply-construct', or
        (setq draft-dir (mu4e~guess-maildir (mu4e-message-field msg :path)))
        (let ((draft-path (mu4e~draft-determine-path draft-dir)))
          (copy-file (mu4e-message-field msg :path) draft-path)
-         (mu4e~draft-open-file draft-path)))
+         (mu4e~draft-open-file draft-path switch-function)))
 
       ((reply forward new)
        ;; case-3: creating a new message; in this case, we can determine
@@ -575,7 +579,7 @@ will be created from either `mu4e~draft-reply-construct', or
                 (reply   (mu4e~draft-reply-construct msg))
                 (forward (mu4e~draft-forward-construct msg))
                 (new     (mu4e~draft-newmsg-construct)))))
-         (mu4e~draft-open-file draft-path)
+         (mu4e~draft-open-file draft-path switch-function)
          (insert initial-contents)
          (newline)
          ;; include the message signature (if it's set)