I hope last public API change.

Since we are storing and caching indexes, and the model used could be filtered/sorted, we want to use QPersistentModelIndex.

I have reached problems of invalid indexes when filtering, and thus having crashes related to this (we cache some indexes => focused, hovered, and more) that result invalid after a filtering.

They still have as .model() the proxy model, but then the proxy model crashes when being called mapToSource().
This avoids those kind of problems.