Add example on how to use Gio::ListModel to populate widgets.

[hugopl]

This example is now crashing, at some point it worked fine, I'm not sure it was a change in GTK libraries that exposed a bug in GICrystal or something else.

Anyway... the crash seems to happen at the interface cast, I experienced issues with interface casts before, I'll let this example here just in case someone are interested in find the issue.

This crash is probably related to why new builds of RTFM also crashes while the old build I have here, with same code as before works.

[hugopl]

Problem found, I'll write it down here so I don't forget.

GICrystal stores the pointer of the Crystal wrapper into the GObject using https://docs.gtk.org/gobject/method.Object.set_qdata.html, so later if the GObject appears it will try to re-use this wrapper if it still exists, this is an optimization for Pure wrappers and a need for Crystal objects that inherit GObject.

However... this optimization is crashing things. If the object first appears as a GObject on Crystal world, it will create a GObject wrapper and store its pointer into GObject C object.

Later we cast this object to its real type, Gtk::ListView in the example, so GICrystal looks at the qdata in the C GObject and retrieves the Crystal wrapper, thinking it's all ok it just do a unsafe cast. Later when you try to use a ListView method in this object it will crash, because it's a Object, not a ListView in Crystal code.

Removing the cache works.

[hugopl]

This example works with hugopl/gi-crystal#133

[hugopl]

BTW the example is failing on CI because of https://gitlab.gnome.org/GNOME/gtk/-/commit/be1729b316da53f12ccc16dc925c563acc2fb4c1

i.e. an old version of SignalListItemFactory used ListItem type instead of GObject in parameters, but it was changed because there are plans to use these factory with other kind of objects.

I'll add a monkey patch in the example itself to deal with this CI situation... or wait until Ubuntu updates their packages.