Add base initialisation function

Add base initialisation function

Add a semi-private function, called clutter_base_init(), which initialises
the basic Clutter functionalities (at the moment, just the GLib type system)
without calling in the backend-specific code. This function is only useful
for gtk-doc, to introspect the library when generating documentation for
signals, properties and class hierarchy.

Also, change the documentation build system to use clutter_base_init() when
launching the scanner program.