script: Allow parsing child properties

script: Allow parsing child properties

The ClutterScript parser needs to be extended to parse child properties
and apply them after an actor has been added to a container. In order to
distinguish child properties from regular GObject properties we can use
the "child::" prefix, e.g.:

  {
    "type" : "ClutterRectangle",
    "id" : "child-01",
    "child::has-focus" : true,
    ...
  }

Parsing child properties can be deferred to the ClutterScriptable
interface, just like regular properties.