Remove superseded SelectionMover class

The SelectionMover class has been superseded by more flexible combinations of filters, scanners and utils. The replacements bring less dependence on the cursor node for positioning, allowing greater flexibility and re-use.

Removing this code and associated tests performs the final step necessary to close #224. In addition, this eliminates redundant line detection logic, so #225 is also closed by this as well.
Good catch. The unit tests added in #747 actually caught this... but the CI server isn't running tests :scream:

In answer to why the ``createParagraphStyleOps`` calls this, it doesn't surprise me you don't recall. This was a (temporary) workaround/hack so that the paragraph AND character styling are preserved when starting a new paragraph. If the cursor is in the last step of a paragraph when the enter key is pressed (to split the paragraph), we capture the character styling at the pre-split cursor node, and create a new automatic style that combines the paragraph + character styling properties for the new paragraph.

Not delightful, but functional enough so far. This was to avoid trying to figure out how to make the cursor move inside an empty span inside an empty paragraph (which it currently doesn't support).