Add FillColor and Transparency properties to the Identifier

The Identifier currently has an OutlineColor property, but not a FillColor property. It renders identified shapes based solely on the user-specified OutlineColor property, applying the same color to a fixed-transparency fill.

It has been requested to add a FillColor property as well, in order to have more control over the rendering. It may also make sense to allow the Transparency level to be customized.