Rename the widget template API

Rename the widget template API

The macros and functions are inconsistently named, and are not tied to
the "template" concept - to the point that it seems plausible to use
them without setting the template.

The new naming scheme is as follows:

  gtk_widget_class_bind_template_child_full
  gtk_widget_class_bind_template_callback_full

With the convenience macros:

  gtk_widget_class_bind_template_child
  gtk_widget_class_bind_template_child_internal
  gtk_widget_class_bind_template_callback

https://bugzilla.gnome.org/show_bug.cgi?id=700898
https://bugzilla.gnome.org/show_bug.cgi?id=700896