Add new Gizmo widget helper to create custom widgets in GTK4

[ARKye03]

While gtkgizmo.c is quite simple, I need to review the code thoroughly and test it in the field (e.g. CircularProgress).

Note

CircularProgress above will be merged onto https://github.com/ARKye03/astal/tree/feat/CircularProgressWidget, to keep work at #282

[ARKye03]

I used it to construct the CircularProgress keeping it more readable and simple, and it works. So, it's ready for review. This widget is no more than a ""copy-paste"" of the gtkgizmo, but in Vala.

[kotontrion]

Should AstalGizmo implement Gtk.Buildable?

No. Creating a Gizmo with a Builder is nonesense, as you can't define the functions that way.

Also using function pointers like here in gobject introspection is not that straight forward. It would involve a bit more work. I think the best cause of action would be making that class internal instead of public, so it can be used by astal, but it's not exposed in the gir.

Otherwise looks good to me, didn't test it though.

[ARKye03]

Should AstalGizmo implement Gtk.Buildable?

No. Creating a Gizmo with a Builder is nonesense, as you can't define the functions that way.

Yes, I realized this rather quickly.

Also using function pointers like here in gobject introspection is not that straight forward. It would involve a bit more work. I think the best cause of action would be making that class internal instead of public, so it can be used by astal, but it's not exposed in the gir.

I made it internal

Now I can rest and ignore GJS bindings <3