Rewrite of line boundary movement actions

Rewrite of line boundary detection logic to address numerous cursor movement issues (#555, #405, #227, #225, #224, #185, #124 and #98).

This also removes the last uses of the gui.SelectionMover class, allowing it to be removed in a subsequent PR.
`edge` -> `edgeOffset`? At least the text in the API dox does not hint to any `edge` (but instead `supplied offset`), so the code reader is slightly confused what `edge` means here.