- Used QtFilterLeafNodes as a proxy model over the real model.
- Added strongly typed QModelIndexes based on StrictModelIndex template.
- Added methods and refactored code to handle conversions between
  different model index spaces correctly - strict QModelIndex classes
  help to enforce this.
- Refactored QtStandardItemTreeAdapter to be a set of methods rather
  than a class - a different logical grouping may follow.
- Added filterRowsBy methods to set the filter predicate and
  resetFilter method to switch to a true-predicate.
- Exposed this filtering functionality to python.
- Split QtTreeCursorNavigation into additional header QtBasicNavigation.
- Moved RowLocation -> QModelIndex and QModelIndex -> RowLocation
  conversion logic into RowLocationAdapter class to allow re-use in other
  contexts such as QtFilterLeafNodes.