Minimize the number of objects created per item in VisualDataModel.

Minimize the number of objects created per item in VisualDataModel.

Derive from QDeclarativeContext and reference count the context object
instead of parenting it to the context.  Combined with a weak
persistent v8 handle the allows the context object to be returned by
javascript accessors instead of creating a new instance for every use.

In addition to the efficiency advantages of creating fewer objects,
routing all data access through a single object means that object can
also persist data instead of just acting as a proxy.

Change-Id: I107dc8c901f16f2a4b420ff1cbffa7a6be27de89
Reviewed-by: Martin Jones <martin.jones@nokia.com>