0.42.1 — Slice pin and config fixes

Fixed

• Slice: rebuilding the sliders no longer throws away the profile pins.
  buildSliceSliders() tears the sliders down and rebuilds them on every
  change, and that teardown deleted every pin placed on a companion strip -- so
  toggling "Link all matching axes" or changing which axes are sliced silently
  lost them, although neither changes what a pin points at. The two cases that
  do are handled where they happen: switching Slice off (no profile left to
  point at) and changing orientation (a pin's index means a sample along the
  other axis afterwards). An axes leaving the scope still drops its own.
• Slice: a null in the embedded option config no longer reads as a number.
  isFinite(null) is true in JavaScript and +null is 0, so panel_size,
  range_min/range_max and index each took a null as a real value --
  panel_size: null collapsed the companion strip onto its minimum size
  instead of the 0.3 default. Figure.to_html() rejects a null before it can
  get there, so this only reached a saved page edited by hand, but the config
  is read the same way either way.