mu4e: context: integrate with the views

Show the current context in headers, view and main mode; add
keybindings.

mu4e/mu4e.el

@@ -35,9 +35,9 @@
 (require 'mu4e-compose)  ;; message composition / sending
 (require 'mu4e-proc)     ;; communication with backend
 (require 'mu4e-utils)    ;; utility functions
+(require 'mu4e-context)  ;; support for contexts
 (require 'mu4e-speedbar) ;; support for speedbar
 
-
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; register our handler functions; these connect server messages to functions