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.
Add clutter_backend_get_n_screens(), which returns the number of screens
manager by the ClutterBackend, and change clutter_backend_get_screen_number()
to return the screen number of the currently used screen; this is semantically
equivalent to clutter_glx_get_default_screen(), but leaves the door open for
a "screen" abstraction in Clutter, handled by the backend. Update the GLX
and EGL backends to retrieve and set the screen_num and n_screens members of
ClutterBackend using Xlib calls.
