Gio::MenuModel: Add signal_items_changed()

* gio/src/menumodel.hg:  Wrap the "items-changed" signal.
gio/src/fileattributeinfo.hg: Remove NO_GTYPE from _WRAP_ENUMs

* gio/src/fileattributeinfo.hg: A comment said that NO_GTYPE was necessary
because gmmproc generated the wrong name for the _get_type() functions.
That was fixed in tools/m4/base.m4 on 2009-03-19 by commit
https://git.gnome.org/browse/glibmm/commit/?id=2343e537edae800a2969d57ec49cb58585b3de22
Probably many more NO_GTYPE can be removed in glibmm.