[build, main-menu] Drop usage of libgnome-2.0 and libgnomeui-2.0

Drop the complete usage of libgnome-2.0 and libgnomeui-2.0
from main-menu.c and remove them from the list of used modules
in configure.in; and call g_set_application_name() instead of
gnome_program_init().

https://bugzilla.gnome.org/show_bug.cgi?id=591001