Fix the definition of GtkWidget:can-target

We are setting the value to TRUE initially, but
the property had a declared default of FALSE.
This is messing up the simplification of .ui files
with gtk4-builder-tool, since it thinks it can
omit can-target properties when it really can't.

gtk/gtkwidget.c

@@ -1013,7 +1013,7 @@ gtk_widget_class_init (GtkWidgetClass *klass)
       g_param_spec_boolean ("can-target",
                             P_("Can target"),
                             P_("Whether the widget can receive pointer events"),
-                            FALSE,
+                            TRUE,
                             GTK_PARAM_READWRITE|G_PARAM_EXPLICIT_NOTIFY);
 
   /**