[66_9] TMU: default to TMU format when Save or Save as

<!-- Thank you for your contribution! -->
## What
+ File -> Save switches to TMU format
+ File -> Save as switches to TMU format
+ `std-S` switches to TMU format
+ `S-F3` switches to TMU format
+ Add support of TM format in the export menu

## How to test your changes?
Tested the shortcuts and menu entries above.

TeXmacs/plugins/tmu/progs/data/tmu.scm

@@ -15,7 +15,7 @@
 
 (define-format tmu 
   (:name "TMU")
-  (:suffix "tmu" "tsu"))
+  (:suffix "tmu"))
 
 (define (texmacs->tmu t)
   (serialize-tmu (cork-tree->utf8-tree t)))

TeXmacs/progs/generic/generic-kbd.scm

@@ -378,7 +378,7 @@
   ("M-S-F2" (new-document*))
   ;;("M-C-F2" (clone-window))
   ("F3" (save-buffer))
-  ("S-F3" (choose-file save-buffer-as "Save TeXmacs file" "texmacs"))
+  ("S-F3" (choose-file save-buffer-as "Save TeXmacs file" "tmu"))
   ("F4" (preview-buffer))
   ("S-F4" (print-buffer))
   ("C-F4" (interactive print-to-file))
@@ -797,7 +797,7 @@
   ("std q" (safely-quit-TeXmacs))
   ("std R" (update-document "all"))
   ("std s" (save-buffer))
-  ("std S" (choose-file save-buffer-as "Save TeXmacs file" "texmacs"))
+  ("std S" (choose-file save-buffer-as "Save TeXmacs file" "tmu"))
   ("std u" (toggle-underlined))
   ("std v" (kbd-paste))
   ("std V" (kbd-paste-verbatim))

TeXmacs/progs/texmacs/menus/file-menu.scm

@@ -139,12 +139,13 @@
 
 (tm-menu (export-menu flag?)
   (with l (converters-from-special "texmacs-file" "-file" #f)
-    (for (fm l)
-      (let* ((name (format-get-name fm))
-             (save-text (string-append "Save " (string-downcase name) " file"))
-             (export-text `(concat "Export as " ,name))
-             (text (if flag? export-text name)))
-        ((eval text) (choose-file (buffer-exporter fm) save-text fm))))))
+    (with l2 (filter (lambda (x) (not (string=? x "tmu"))) l)
+      (for (fm l2)
+        (let* ((name (format-get-name fm))
+               (save-text (string-append "Save " (string-downcase name) " file"))
+               (export-text `(concat "Export as " ,name))
+               (text (if flag? export-text name)))
+          ((eval text) (choose-file (buffer-exporter fm) save-text fm)))))))
 
 (tm-define (export-top-menu) (export-menu #t))
 (tm-define (export-export-menu) (export-menu #f))
@@ -181,7 +182,7 @@
 
 (menu-bind save-menu
   ("Save" (save-buffer))
-  ("Save as" (choose-file save-buffer-as "Save TeXmacs file" "texmacs"))
+  ("Save as" (choose-file save-buffer-as "Save TeXmacs file" "tmu"))
   ---
   (link export-top-menu)
   ---
@@ -270,7 +271,7 @@
         ("Clear menu" (forget-interactive "recent-buffer"))))
   ---
   ("Save" (save-buffer))
-  ("Save as" (choose-file save-buffer-as "Save TeXmacs file" "texmacs"))
+  ("Save as" (choose-file save-buffer-as "Save TeXmacs file" "tmu"))
   ---
   (link print-menu)
   ---
@@ -281,6 +282,7 @@
   (-> "Export"
       (link export-export-menu)
       ---
+      ("TM document" (choose-file save-buffer-as "Save TeXmacs file" "texmacs"))
       ("Pdf" (choose-file wrapped-print-to-file "Save pdf file" "pdf"))
       ("Pdf with embedded document" (choose-file wrapped-print-to-pdf-embeded-with-tm "Save pdf file" "pdf"))
       ("Postscript"

TeXmacs/progs/texmacs/texmacs/tm-files.scm

@@ -190,7 +190,7 @@
     (cond ((url-scratch? name)
            (choose-file
              (lambda (x) (apply save-buffer-as-main (cons x opts)))
-             "Save TeXmacs file" "texmacs"))
+             "Save TeXmacs file" "tmu"))
           ((not (buffer-exists? name))
            (with msg `(concat "The buffer " ,vname " does not exist")
              (set-message msg "Save file")))