Add a getter for the number of children

Having this api will let us optimize things later.
For now, just iterate over the children.

docs/reference/gtk/gtk4-sections.txt

@@ -4631,6 +4631,7 @@ gtk_widget_get_next_sibling
 gtk_widget_get_prev_sibling
 gtk_widget_get_first_child
 gtk_widget_get_last_child
+gtk_widget_get_n_children
 gtk_widget_insert_before
 gtk_widget_insert_after
 

gtk/gtkwidget.c

@@ -15306,6 +15306,28 @@ gtk_widget_get_last_child (GtkWidget *widget)
   return widget->priv->last_child;
 }
 
+/**
+ * gtk_widget_get_n_children:
+ * @widget: a #GtkWidget
+ *
+ * Returns: the number of children
+ *
+ * Since: 3.92
+ */
+guint
+gtk_widget_get_n_children (GtkWidget *widget)
+{
+  guint n = 0;
+  GtkWidget *child;
+
+  for (child = gtk_widget_get_first_child (widget);
+       child != NULL;
+       child = gtk-widget_get_next_sibling (child))
+    n++;
+
+  return n;
+}
+
 /**
  * gtk_widget_get_next_sibling:
  * @widget: a #GtkWidget

gtk/gtkwidget.h

@@ -1170,6 +1170,8 @@ GDK_AVAILABLE_IN_3_90
 GtkWidget *             gtk_widget_get_first_child      (GtkWidget *widget);
 GDK_AVAILABLE_IN_3_90
 GtkWidget *             gtk_widget_get_last_child       (GtkWidget *widget);
+GDK_AVAILABLE_IN_3_92
+guint                   gtk_widget_get_n_children       (GtkWidget *widget);
 GDK_AVAILABLE_IN_3_90
 GtkWidget *             gtk_widget_get_next_sibling     (GtkWidget *widget);
 GDK_AVAILABLE_IN_3_90