Add grid snapping to graph imports/exports; improve layer panel drag into/between insertion; better preserve graph space on reordering

[adamgerhant]

Reworks the default positioning system when moving nodes in the layer panel, mostly by ensuring that node gap positioning is correctly maintained. Each layer is the height of that layer plus the height to the next layer below. Fixes all edge cases when moving layers or different groups of selected layers to any other location. Also improves the previewing system and import/export edges

Todo:

• Create space when moving layers to different stacks

[Keavon]

A couple more edge cases when snapping needs to get engaged:

• Scrolling with your mouse's horizontal mouse scroll wheel (or Shift+vertical scroll wheel)
• Scrolling with the horizontal scroll bar

[Keavon]

Ready to merge after you fix that edge case.

[Keavon]

One more very minor, harmless edge case I found: when you open a demo artwork, then open the graph, its export is in the wrong place before you pan for the first time.