This is like a GtkModelButton, but with a custom child inside, which
isn't currently possible with GtkModelButton.

As a GtkButton sub-class, ideally, GtkModelButton should allow users to
override its internals with a custom child, but currently it doesn't.
It's also not possible, at least not easily, to override its
appearance using the "role" property in some cases. For example, if a
stateful GAction should be represented by a normal button instead of a
radio button. One could fix GtkModelButton, but given it's intended
purpose of being a semi-private widget that's used to bind GtkPopovers
to GMenuModels, it's not worth the added complexity.

#29