58359238 make reordering discoverable

Adds a drag handle icon that appears when hovering items in the picker and TOC (hidden with CSS for *books* in the picker). The title/tooltip text for the handle 1) is non-existent for current-book items in the picker, 2) mentions copying for non-current-book items in the picker, or 3) is just "Drag to a different part of the book" in the TOC. 

Several styling/spacing changes were made along the way for the trees.