mu4e-view: re-enable the gnus/old split

re-enable the gnus/old split (the key was re-implementing
mu4e-view-message-text for gnus)

split helpers into mu4e-view-common

as a bonus 'go to url' now also works with gnus

mu4e/Makefile.am

@@ -40,11 +40,12 @@ dist_lisp_LISP=			\
 	mu4e-utils.el		\
 	mu4e-vars.el		\
 	mu4e-view.el		\
+	mu4e-view-common.el	\
+	mu4e-view-gnus.el	\
+	mu4e-view-old.el	\
 	mu4e.el			\
 	obsolete/org-mu4e.el
 
-#	mu4e-view-gnus.el
-#	mu4e-view-old.el
 
 EXTRA_DIST=			\
 	mu4e-about.org