* clutter/clutter-overview.xml: Split the overview into its own
file, so we gtk-doc can linkify all class names.
* clutter/building-clutter.xml: Split the building instructions
into its own file, for better maintainability.
* clutter/clutter-animation.xml:
* clutter/creating-behaviours.xml:
* clutter/subclassing-ClutterActor.xml: Rename from SGML to XML;
these were not SGML files anyway, but templates.
* clutter/clutter-docs.sgml: Use XInclude instead of the ugly
entities hack.
* clutter/Makefile.am: Update the build.
