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.
In particular gtksettings.h and gtkstylecontext.h needed to be included
in lots of places now.
Also, I order the includes alphabetically in a bunch of headers.
