X11: Reuse menu code to display files

Previously the code used the ASCII Text Athena widgets for displaying
file contents. Unfortunately, the widget made it impossible to control
scrolling or pretty much anything else.

Use the menu code instead, making the file display window behave properly.

include/winX.h

@@ -202,6 +202,7 @@ struct xwindow {
     XColor nh_colors[CLR_MAX];
     XFontStruct *boldfs;       /* Bold font */
     Display *boldfs_dpy;       /* Bold font display */
+    char *title;
 
     union {
         struct map_info_t *Map_info;       /* map window info */