Adjust StepsTranslator behaviour

When the requested number of steps passed to convertStepsToDomPoint is
beyond the document end in, or before the start of the document, an
exception will be thrown. This closes a slight hole where asking for a
step beyond the document would return a non-step position. It also makes
the invalid usage of the interface more obvious.
Reducing usages of SelectionMover.countSteps

SelectionMover.countSteps has been replaced by step iterators and other
more performant solutions.
Remove last usages of OdtCursor.move from tests

All remaining OdtCursor tests are really testing other parts of the system that
are already covered in their own unit tests. For now, this file has been removed
and should be replaced later when OdtCursor gains additional tests.
Remove SelectionMover functions that are only used in tests

There are no worthwhile unit tests in SelectionMoverTests after these
are cleaned out, so for now the whole class file has been removed.

Mid-term, the SelectionMover class is to be removed as soon as an
implementation of navigate-by-line-boundaries has been created for
the step-iterator approach.