Better dragging behavior

[edemaine]

It's currently hard to select objects and drag them, because the drag needs to exactly hit an object. Proposed alternate behavior [Josh]:

• [x] If you drag anywhere within the dashed bounding box, then it moves the objects.
• [x] However, if you do a click operation below the drag threshold, it behaves as is (so can still easily select a different object or deselect).
• [x] Also, shift-clicks behave as before, so it's easy to add/remove items (including with rectangular select).
• [x] Ideally, we highlight the box when hovering over it, and no shift key is held, to give a visual indicator that dragging the box is that easy.

Probably replaces #109.