Create marker start and end

[dschuyler]

I was looking into a bug with line selection at the end of the document and noticed that in a couple other gui editors the selection seems to be more than just the pen and marker. To behave in a similar way, ci may need a start and an end marker, aside from the pen.

This will add two values to 'm' redo nodes. I've actually added a marker end in the past and then removed it as a space optimization.

An alternative may be adding more states to the selectionMode. This needs more exploration to see if it's workable.

A second alternative is to track markers in a more transitory way (something akin to tempChange) so that it's not in the redo chain. It may not be necessary to record an extra row, col pair.