Also:

- some more experiments with Three.js on slides performed
- some unused functions removed

The use of libsass.dso somehow prevented GHCi to compile and run decker.
That in turn made using of intero impossible. Which is not an option.

Increase gap between columns to ensure that left rounded borders of highlighted boxes in the right column are not overlaid by the left column

But only for development versions.

Also remove the resources in `~/.local/share` when `clean` is performed
on a development version.

Style sheets can now be added in CSS or SCSS format. SCSS files are
automatically compiled to CSS and added as a dependency to whatever
target uses them. Which means changes to SCSS files are picked up by
`decker watch` and `decker server`.