SVG Overlay: remove debug output (minor)
SVG Overlay: update comment (minor)
SVG Overlay: correct middle click event handling

Also reformat handlers for clarity and display correct node ID in access
error message.
SVG Overlay: prevent text selection when dragging

D3 handles event propagation and default dragging behavior separately.
Disable both when dragging a node to prevent text labels and widget text
from being highlighted.
SVG Overlay: always scale new stacks

Scaling when sometimes not being applied when first opening the tracing
tool because the doNotUpdate flag was set. Have the overlay layer always
call the SkeletonElements scale function, and make the scale function
responsible for determining whether it actually needs to rescale to
avoid unnecessary SVG manipulation.
SVG Overlay: update scale as settings are adjusted