Refactor QQuickVisualAdaptorModel to reduce memory consumption.

Refactor QQuickVisualAdaptorModel to reduce memory consumption.

Don't inherit from QObject.
Use a single QQmlGuard for all model types, and reset the model
property if the model is deleted.
Construct v8 object template on demand.
Store model type specific data in a separate class that is allocated
on demand.

Change-Id: Id4f7b235741555b6ffba3fcf11727d85d6920e9e
Reviewed-by: Martin Jones <martin.jones@nokia.com>