Anchors and snapping improvements

[avvvvve]

1. In page view, a hairpin at the end of one system followed by a dynamic at the beginning of the next system don't act "snapped" together, even though they do when in continuous view.
2. Shift + arrow on a hairpin with a dynamic at the end/beginning should move the dynamic with it when snapping is turned on
3. Shift + arrow doesn't work on hairpins split across systems
4. Dragging a dynamic from one system to another is weird. Ideally, if your mouse position gets close to a different system, the element would start previewing there instead of on the current system.