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>