Merge changes from topic 'qaim-dfs-iterator'

Merge changes from topic 'qaim-dfs-iterator'

* changes:
  Refactor & test moving to next/previous unread messages
  Gui: use QaimDfsIterator when checking for previous/next unread messages
  Check reverse iteration of a QaimDfsIterator
  stashing_reverse_iterator: Fix a build error
  Import a version of reverse_iterator which supports stashing iterators
  Check that QaimDfsIterator's operator++ is reversible
  Backward iteration through a QAbstractItemModel
  Support iteration up until a given top-level index
  Iterate through backtracking to the upper layers of the QAIM tree
  Support iterating through a simple, linear depth of children
  Iterator for walking through indexes within a flat QAIM