Snap-to-grid

[bergie]

Currently node dragging is 100% accurate, following your pointer (though, see #5). This makes graphs fiddly and easily messy-looking. We should instead implement a snap-to-grid feature. The sizing of that grid is another question... even a 10 pixel grid could be helpful, but on the bigger end we could even impose a grid of the same size as the nodes are.

[forresto]

Zoom level is a consideration here too. It would be easier to snap to grid when zoomed all the way out if grid size = node size.

[forresto]

snapped clock graph

[bergie]

Snap-to-grid doesn't seem to be working in SVG branch, @forresto

[forresto]

I haven't put it in, thinking that it would conflict with autolayout.

@automata can klay return xy values that are all %36?

If not, it could be a good time to make it snap to other nodes ... that would work with klay's layered approach.