Add tests for the StepIterator class
Rewrite image selection to use StepIterators

OdtDocument.getDistanceFromCursor is no longer used by any code.
Rewrite some cursor move & extend actions to use a StepIterator
Simplify PositionFilterChain to not use named filters

PositionFilterChains no longer have any need to allow adding or removal
of named filters. In practice, filters are only ever added just after
construction, and remain unchanged beyond this point. This allows a
slight simplification of the PositionFilterChain interface.