Omit displaying anchor/handle's snapping candidate point for the dragged point

[Keavon]

From here:

When dragging an anchor point, can we ~~possibly (without too much effort or blockage before merging #617)~~ skip the own anchor + both handles from being candidate snapping points? And when dragging a handle, skip just the handles. Unless they're split, then just the own handle but its opposing handle which isn't moving should be a candidate. Or in simpler terms: if the anchor/handle point is actively moving, don't use its former position as a candidate snapping point.