Core API: comp value convenience accessors (#418).

7 new convenience accessors with "defaultValue" argument.
These are handy when your plugin tolerates a missing child topic.
No "Invalid Access" execption is thrown. Instead the default value is returned.

CompositeValue public API:
    - String         getString(   String childTypeUri, String         defaultValue)
    - int            getInt(      String childTypeUri, int            defaultValue)
    - long           getLong(     String childTypeUri, long           defaultValue)
    - double         getDouble(   String childTypeUri, double         defaultValue)
    - boolean        getBoolean(  String childTypeUri, boolean        defaultValue)
    - Object         getObject(   String childTypeUri, Object         defaultValue)
    - CompositeValue getComposite(String childTypeUri, CompositeValue defaultValue)

CompositeValueModel public API:
    - String              getString(   String childTypeUri, String              defaultValue)
    - int                 getInt(      String childTypeUri, int                 defaultValue)
    - long                getLong(     String childTypeUri, long                defaultValue)
    - double              getDouble(   String childTypeUri, double              defaultValue)
    - boolean             getBoolean(  String childTypeUri, boolean             defaultValue)
    - Object              getObject(   String childTypeUri, Object              defaultValue)
    - CompositeValueModel getComposite(String childTypeUri, CompositeValueModel defaultValue)

Close ticket 418.