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.
Specifically, I was going for what could be best described as a "memento" pattern here :hamburger: http://en.wikipedia.org/wiki/Memento_pattern