mu4e: make global-mode-string buffer-local

Make `global-mode-string' local in all buffers where we use it, so we
don't leak the "context" string outside mu4e.

Also add the context string to the compose buffer's modeline.

This fixes #740.

mu4e/mu4e-compose.el

@@ -288,6 +288,9 @@ message-thread by removing the In-Reply-To header."
 \\{message-mode-map}."
   (progn
     (use-local-map mu4e-compose-mode-map)
+
+    (set (make-local-variable 'global-mode-string) '(:eval (mu4e-context-label))) 
+   
     (set (make-local-variable 'message-signature) mu4e-compose-signature)
     ;; set this to allow mu4e to work when gnus-agent is unplugged in gnus
     (set (make-local-variable 'message-send-mail-real-function) nil)

mu4e/mu4e-main.el

@@ -70,8 +70,12 @@
   "Major mode for the mu4e main screen.
 \\{mu4e-main-mode-map}."
   (use-local-map mu4e-main-mode-map)
-  (setq truncate-lines t
-        overwrite-mode 'overwrite-mode-binary)
+  (setq
+    truncate-lines t
+    overwrite-mode 'overwrite-mode-binary)
+
+  ;; show context in mode-string
+  (set (make-local-variable 'global-mode-string) '(:eval (mu4e-context-label))) 
   (set (make-local-variable 'revert-buffer-function) #'mu4e~main-view-real))
 
 

mu4e/mu4e-view.el

@@ -734,10 +734,11 @@ FUNC should be a function taking two arguments:
   (make-local-variable 'mu4e~view-attach-map)
   (make-local-variable 'mu4e~view-cited-hidden)
 
-  (setq buffer-undo-list t ;; don't record undo info
-    global-mode-string     ;; show context in mode-line
-    '(:eval (mu4e-context-label))) 
-  
+  ;; show context in mode-string
+  (set (make-local-variable 'global-mode-string) '(:eval (mu4e-context-label))) 
+ 
+  (setq buffer-undo-list t);; don't record undo info
+     
   ;; autopair mode gives error when pressing RET
   ;; turn it off
   (when (boundp 'autopair-dont-activate)