Add a new class, GtkMenuTrackerItem that represents a menu item, to be
used with GtkMenuTracker.

GtkMenuTracker's insert callback now works in terms of this new type
(instead of passing reference to the model and an index to the item).

GtkMenuShell now handles all of the binding tasks internally, mostly
through the use of property bindings.  Having bindings for the label and
visibility attributes, in partiular, will help with supporting upcoming
extensions to GMenuModel.

GtkModelMenu has been reduced to a helper class that has nothing to do
with GMenuModel.  It represents something closer to an "ideal" API for
GtkMenuItem if we didn't have compatibility concerns (eg: not emitting
"activate" when setting toggle state, no separate subclasses per menu
item type, supporting icons, etc.) Improvements to GtkMenuItem could
eventually shrink the size of this class or remove the need for it
entirely.

Some GtkActionHelper functionality has been duplicated in
GtkMenuTracker, which is suboptimal.  The duplication exists so that
other codebases (such as Unity and gnome-shell) can reuse the
GtkMenuTracker code, whereas GtkActionHelper is very much tied to
GtkWidget.  Supporting binding arbitrary GtkWidgets to actions vs.
supporting the full range of GMenuModel features for menu items turns
out to be two overlapping but not entirely similar problems.  Some of
the duplication (such as roles) can be removed from GtkActionHelper once
Gtk's internal Mac OS menubar support is ported to GtkMenuTracker.

The intent to reuse the code outside of Gtk is also the reason for the
unusual treatment of the enum type introduced in this comment.

This adds no new "public" API to the Gtk library, other than types that
we cannot make private due to GType limitations.