interval: Implement ClutterScriptable interface

This allows the creation of ClutterTransition objects in ClutterScript:
 {
   "id" : "scripted-transition",
   "type" : "ClutterPropertyTransition",
   "property-name" : "background-color",
   "interval" : {
     "type" : "ClutterInterval",
     "value-type" : "ClutterColor",
     "initial" : "red",
     "final" : "blue"
   }
 }
interval: Call g_object_set_property in set_custom_property()

Otherwise it would prevent potential subclasses of ClutterInterval from
having own scriptable properties.