Introduce GtkMenuTracker

Introduce GtkMenuTracker

GtkMenuTracker folds a nested structure of sections in a GMenuModel into
a single linear menu, which it expresses to its user by means of 'insert
item at position' and 'remove item at position' callbacks.

The logic for where to insert separators and how to handle action
namespaces is contained within the tracker, removing the need to have
this logic duplicated in the 3 or 4 places that consume GMenuModel.

In comparison with the previous code, the tracker no longer completely
destroys and rebuilds menus every time a single change occurs.  As a
result, the new gtkmenu testcase now runs in approximately 3 seconds
instead of ~60 before.

https://bugzilla.gnome.org/show_bug.cgi?id=696468