Lets the tree ref the node.
This is an optional method for models to implement. To be more specific, models may ignore this call as it exists primarily for performance reasons.
This function is primarily meant as a way for views to let caching models know when nodes are being displayed (and hence, whether or not to cache that node). Being displayed means a node is in an expanded branch, regardless of whether the node is currently visible in the viewport. For example, a file-system based model would not want to keep the entire file-hierarchy in memory, just the sections that are currently being displayed by every current view.
A model should be expected to be able to get an iter independent of its reffed state.