Follow-up to #721:

- Move the toggle out of Advanced Settings into a real preference
  (Settings > Appearance, kPreferenceKeyScrollableSideTabBar) with a
  checkbox in PreferencePanel.xib.
- Drag-to-reorder honors the scroll offset (new verticalScrollOffset
  accessor) so dragging while scrolled no longer snaps cells or drops
  on the wrong index.
- Newly created tabs, and any tab that becomes active, scroll fully
  into view below the window decorations rather than under them.
- Keep tab content and accessory subviews (spinner, progress bar) from
  drawing under the window decorations, using a drawing clip rather
  than clipsToBounds so window transparency is preserved.
- Re-sync hover/highlight after scrolling so cells don't stay stuck
  highlighted when they move out from under the cursor.