* clutter/Makefile.am:
	* clutter/clutter.h:
	* clutter/clutter-scriptable.[ch]: Add the ClutterScriptable
	interface; by implementing this interface, a class can
	override the UI definition parsing and transform complex data
	types into GObject properties, or allow custom properties.

	* clutter/clutter-script.c:
	* clutter/clutter-script-parser.c:
	* clutter/clutter-script-private.h: Rearrange the code and
	use the ClutterScriptable interface to parse and build the
	custom properties. This cleans up the code and also it makes
	it more reliable (the complex type parsing is now done using
	the target type and not just the name of the property).