Some tidying up and Snappoints#1

I tidied up how the drag and drop is handled, moving it to a singleton class, rather than trying to work it out in each sheet individually, this enabled me to fix a problem where things went wrong if you moved the mouse across a sheet while dragging another one.  Basically it is necessary to lock the focus sheet to the one being dragged, for as long as the drag goes on, but then correctly pick up the change in focus when the drag is over...

And I also added snap-points.  Nothing is snapping to them yet, but I worked out where they needed to be, alongside the text that needs snapping to, and I have (temporary) code to display them as little red and green arrows.  There's two kinds of snap-point, because left text only snaps onto right text, and vice-versa...