event-editor/multi-choice: Inherit GtkWidget

This is a prerequisite for removing the strong dependence on the
child GtkToggleButton widget, removing the platform state override,
and adding binding actions in later commits.

Inheriting AdwBin was attempted but it didn't work, as AdwBin
presumably messes with focusability and activation, making
it insufficient for this use case.

`hexpand=true` needs to be explicitly set.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/841>

src/gui/event-editor/gcal-multi-choice.c

@@ -26,7 +26,7 @@
 
 struct _GcalMultiChoice
 {
-  GtkBox                          parent;
+  GtkWidget                       parent;
 
   GtkWidget                      *down_button;
   GtkWidget                      *button;
@@ -82,7 +82,7 @@ static void gcal_multi_choice_accessible_init (GtkAccessibleInterface *iface);
 
 static void gcal_multi_choice_accessible_range_init (GtkAccessibleRangeInterface *iface);
 
-G_DEFINE_FINAL_TYPE_WITH_CODE (GcalMultiChoice, gcal_multi_choice, GTK_TYPE_BOX,
+G_DEFINE_FINAL_TYPE_WITH_CODE (GcalMultiChoice, gcal_multi_choice, GTK_TYPE_WIDGET,
                                G_IMPLEMENT_INTERFACE (GTK_TYPE_ACCESSIBLE, gcal_multi_choice_accessible_init)
                                G_IMPLEMENT_INTERFACE (GTK_TYPE_ACCESSIBLE_RANGE, gcal_multi_choice_accessible_range_init))
 
@@ -335,6 +335,8 @@ gcal_multi_choice_dispose (GObject *object)
 {
   GcalMultiChoice *self = GCAL_MULTI_CHOICE (object);
 
+  g_clear_pointer ((GtkWidget **) &self->popover_bin, gtk_widget_unparent);
+
   g_clear_pointer (&self->choices, g_free);
   g_clear_pointer (&self->category, g_free);
   g_clear_pointer (&self->prev_button_tooltip_text, g_free);
@@ -608,6 +610,8 @@ gcal_multi_choice_class_init (GcalMultiChoiceClass *class)
   gtk_widget_class_bind_template_callback (widget_class, key_pressed_cb);
 
   gtk_widget_class_set_css_name (widget_class, "navigator");
+
+  gtk_widget_class_set_layout_manager_type (widget_class, GTK_TYPE_BIN_LAYOUT);
 }
 
 static void