You would expect a widget parent to be a Container, yet the APIs have them
as ordinary Widgets.  This leads to excessive casting.

void reparent(Widget & newparent);
Widget* get_parent();
const Widget* get_parent() const;
void on_parent_changed(Widget* previous_parent);
Glib::SignalProxy1<void,Widget*> signal_parent_changed();
void set_parent(Widget& parent);

Perhaps oddly, property_parent() is a Container.

Obviously other than the get_parent calls, this change would break API
compatibility, but only for broken programs anyway.

Perhaps also get_parent should have covariant returns - I would expect
MenuItem::get_parent() to return a MenuShell for example.