Circuit editor multi-register operation dragging about q.js

[stewdio]

This is 90% resolved 😄

A user can now drag multi-register operations all around! Drag the whole thing. Drag part of it. Drag pieces within the same moment. Drag one component onto a sibling component (thereby erasing the sibling underneath). It works!

But before closing this we ought to update Q.Circuit.prototype.clear$ to be kinder to existing multi-register operations. Right now clearing one cell that happens to be occupied by one component of a multi-reg op will erase the whole thing. That’s not so much fun. We ought to break it apart and just erase the piece that was cleared.