This is a neat trick to get around the circularity between GDK, GSK and
GTK that we inherit with the GdkPaintable interface.
GdkPaintable uses GtkSnapshot
GtkSnapshot creates GskRenderNodes
GskRenderNodes use GdkTextures
GdkTexture will soon implement GdkPaintable
This causes a loop that spans GDK, GSK and GTK and this is the easiest
way to resolve it without breaking bindings (at least that's the idea).
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.
