Add Composite Child machinery and APIs to GtkWidget
This commit implements the needed machinery for GtkWidget to build it's composite content from GtkBuilder XML and adds the following API: o gtk_widget_init_template() An api to be called in instance initializers of any GtkWidget subclass that uses template XML to build it's components. o gtk_widget_class_set_template() API to associate GtkBuilder XML to a given GtkWidget subclass o gtk_widget_class_automate_child() API to declare an object built by GtkBuilder to be associated with an instance structure offset and automatically set. o gtk_widget_get_automated_child() API for bindings to fetch a child declared to be automated by gtk_widget_class_automate_child(), for the case where bindings do not generate GObjects under the hood and cannot use structure offsets to resolve composite object pointers. o gtk_widget_class_declare_callback[s]() Declare static functions to be used in signal callbacks from a given class's template XML o gtk_widget_class_set_connect_func() API for bindings to override the signal connection machinery for a given GtkWidget derived class.
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