Remove parts of the Obsolete SelectionMover interface

For various performance reasons, the SelectionMover class has been largely replaced by step iterators. This commit removes a bunch of features used only in tests, either adapting the test where still useful, or removing the test entirely when there is no further functionality in the test.

One small thing to note is that this PR adjusts the StepsTranslator behaviour whenthe requested number of steps is beyond the document end. convertStepsToDomPoint will now return the last walkable step
rather than the current iterator position. This closes a slight hole where asking for a step beyond the document would return a non-step position.