model: Implement ClutterScriptable interface

model: Implement ClutterScriptable interface

Allow a ClutterModel to be constructed through the ClutterScript API.
Currently this allows a model to be generated like like this:

{
  "id" : "test-model",
  "type" : "ClutterListModel",
  "columns" : [
    [ "text-column", "gchararray" ],
    [ "int-column", "gint" ],
    [ "actor-column", "ClutterRectangle" ]
  ]
}

where 'columns' is an array containing arrays of column-name,
column-type pairs.

http://bugzilla.openedhand.com/show_bug.cgi?id=2007