The information below guided the units:
CastleWindowRecentFiles, CastleLclRecentFiles, CastleRecentFiles.

------------------------------------------------------------------------------
How should recent files menu look and work like in GNOME program ?

(I didn't found this info in any guideline on Internet,
especially not in GNOME HIG).

Inline "recent files" menu:
- Examples: See gedit, eog, evince, totem
- Size: At most 5 recent items
- Placement: After "print" commands, there's a separator,
  then a recent files list, then separator and "close" commands.
  Menu items are sorted from 1st (the most recent one).
- Look: "n" + ". " + basename (with extension, without path) of the filename.
  "n" is between 1 and 5, and it's a key shortcut to this menu item.
  Only the basename is shown, to see full filename you should move to this
  menu item --- gedit will show full filename in status bar,
  eog will show it as a hint, in both cases the text is
    Opens "full-filename"

--------------------
Separate "recent files" menu:
- Examples: See gimp
- Size: At most 10 recent items
- Placement: As a submenu of "Open recent" in main menu.
  "Open recent" is right below "Open location", so it's visually
  connected with other "Open ..." commands.
  Menu items are sorted from the most recent one.
- Look: just a file basename. There is no underlined menu shortcut,
  but there is a global shortcut: Ctrl+1, Ctrl+2 etc.
  The last (10th) item has a shortcut Ctrl+0.
- The "Open recent" submenu ends with separator and command
  "Document History" that displays a window with full (long) list
  of recent files, allowing you to open files on this list and do
  some other operations.

--------------------
There are reasons after and against separate "recent files",
see e.g. [http://bugzilla.gnome.org/show_bug.cgi?id=349541]
or [http://log.emmanuelebassi.net/archives/category/hacking/recent-files/].
In short:
+ inline recent files menu allows you to avoid submenus, and avoiding
  submenus makes it more visible and easier to navigate
- inline recent files menu blocks you from placing too many items
  there ("File" menu would grow too large)
- inline recent files menu doesn't give you a clear place where you
  could place an item like "More" ("Document History" known from GIMP).

--------------------
Somewhat related to this is
[http://standards.freedesktop.org/recent-file-spec/recent-file-spec-0.2.html]
but it seems to be used only by GIMP for now.