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.

clutter/clutter-script-private.h

@@ -78,6 +78,8 @@ typedef struct {
   gchar *name;
   JsonNode *node;
   GParamSpec *pspec;
+
+  guint is_child : 1;
 } PropertyInfo;
 
 typedef struct {