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.
Not sure if one should just assume the units of the offset to be in pixel, given the method name. But I would prefer to be explicit about that in the API dox.