Add internal refs property support

[CADBIMDeveloper]

Several model properties (such as Level property of the Revit model) could be internal refs. Attribute definition of such property is something like:

{
    "name": "Level",
    "category": "__internalref__",
    "dataType": 11,
    "dataTypeContext": null,
    "description": null,
    "displayName": null,
    "flags": 1,
    "precision": 0
}

Property database getAttrValue method returns dbId of corresponding ref element. We should get name property of that object