Gio: Menu: Add insert/append/prepend_section() with no label parameter.
Gio: Menu: Add insert/append/prepend_section() with no label parameter.

* gio/src/menu.hg: Because the label may be NULL in C,
  meaning that the sections will just be separated by spaces
  instead of having titles in the menu.
  Bug #702456 .

johne53/MB3Glibmm,GNOME/glibmm,johne53/MB3Glibmm,johne53/MB3Glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,johne53/MB3Glibmm