mu4e-window: disable functionp mu4e-split-window for now

Set it to 'horizontal. It didn't quite work with the new window handling
code, such that mu4e was not functional. So let's restore the basics at
least.

mu4e/mu4e-window.el

@@ -235,10 +235,14 @@ modifying `display-buffer-alist'.
 
 If `mu4e-split-view' is a function, then it must return a live window
 for BUFFER-OR-NAME to be displayed in."
-  (if (functionp mu4e-split-view)
-      (set-window-buffer (funcall mu4e-split-view) buffer-or-name)
+  ;; For now, using a function for mu4e-split-view is not behaving well
+  ;; Turn off.
+  (when (functionp mu4e-split-view)
+    (mu4e-message "Function for `mu4e-split-view' not supported; fallback")
+    (setq mu4e-split-view 'horizontal))
+
   (let* ((buffer-name (or (get-buffer buffer-or-name)
-                            (error "Buffer `%s' does not exist" buffer-or-name)))
+                          (mu4e-error "Buffer `%s' does not exist" buffer-or-name)))
          (buffer-type (with-current-buffer buffer-name (mu4e--get-current-buffer-type)))
          (direction (cons 'direction
                           (pcase (cons buffer-type mu4e-split-view)
@@ -277,7 +281,7 @@ for BUFFER-OR-NAME to be displayed in."
                 )))
     (funcall (if select #'pop-to-buffer #'display-buffer)
              buffer-name
-               arg))))
+             arg)))
 
 (defun mu4e-resize-linked-headers-window ()
   "Resizes the linked headers window belonging to a view.