Attribute Read Resource Reorder

pqSMTKDisplayAttributeOnLoadBehavior::handleResourceEvent() was triggering the display of an empty Attribute Resource because our smtk::attribute::Read runs on a non-main thread. A race condition arises where the single-shot timer in case EventType::ADDED runs sooner than the from_json() returns, since the main thread event loop is free, causing an empty Attribute Resoure to be displayed.

To remedy this, an unmanaged Attribute Resource is created, then added to the Resource Manager after from_json() completes on the worker thread. This way, handleResourceEvent() has something to display. Resolving Resource Links has been removed, because Manager::Add() performs that task.