Investigate replacing derived classes with Qt standards

Currently, all of our model/view/item classes are derived from a Qt abstract class.
We could maybe replace some with Qt standard concrete implementations instead of rolling our own.

The issue names the model/view/item classes but no files or tests. Start by locating those classes and checking which Qt abstract classes they derive from, then compare each with available Qt concrete implementations. Done means documenting a clear replacement scope and the affected classes.

Written by the indexing model from the issue text.