mu4e-view: create dummy summary buffer

Some Gnus commands depend on the existence of a *Summary* buffer; and in
older versions of mu4e we created it, since mu4e depended on it. It no
longer does, but some gnus command users may want to try, do.

So create the Summary buffer once again.

mu4e/mu4e-view.el

@@ -474,6 +474,12 @@ As a side-effect, a message that is being viewed loses its
       (if (memq mu4e-split-view '(horizontal vertical))
           (delete-windows-on existing-buffer t))
       (kill-buffer existing-buffer))
+
+    ;; HACK: we create a *Summary* buffer, since its mere existence
+    ;; is enough for some gnus commands / functions to work in
+    ;; in the mu4e view as well.
+    (get-buffer-create (or gnus-summary-buffer "*Summary*"))
+
     (setq gnus-article-buffer (mu4e-get-view-buffer nil t))
     (with-current-buffer gnus-article-buffer
       (when linked-headers-buffer