Add an v8 object template for visual data model change sets.

Add an v8 object template for visual data model change sets.

This is more optimal than dynamically creating an object per
change for each signal emission.  Also create static string
instances for property names common to all item objects.

Change-Id: I8b0a6b683df4ee9c55c15cc928603e82f37045f7
Reviewed-by: Martin Jones <martin.jones@nokia.com>
Reviewed-by: Chris Adams <christopher.adams@nokia.com>