Cursor / Selection API changes in Editor

Here are my suggestions for editing the cursor and selection API. My main goal is consistency and symmetry. I'll pepper with inline comments explaining my decisions.
I've renamed all these to remove the `Cursor` name. I want it to be clear that this is moving _all_ the cursors, not just one. We could say `moveCursors*`? But I think this matches with `selectTo` in the selection API.