Correcting two typos.
Correcting links tow the API documentation
Use the generic function gtk-menu-item-submenu
Correcting index entries and links to the API documentation
Correcting some links.
Removing the functions:

gtk-menu-item-get-submenu,
gtk-menu-item-set-submenu
gtk-menu-item-get-label
gtk-menu-item-set-label.

These functions are implemented as generic functions.
Correcting links to the API documentation
Correcting links to the API documentation
Work on the chapter for menus
Removing some links.
Work on the implementation of GtkEntry