Catsop: factor assembly overlay into layer

Factor assembly overlay into a layer extending CatsopResultsLayer. While
open, this layer automatically refreshes when z-section changes, but
must be manually refreshed to reload slices when the view window pans.
The CATSOP widget now tracks layers it has opened by a name, which can
be turned into a stack layer key with getLayerKey.
Catsop widget: properly destroy layers on close
Catsop widget: make overlay toggling generic

Move assembly overlay to a widget tab as a checkbox. Make this the
generic approach for toggling extensions to CatopResultsLayer.