Jupyterlab-gridwidth

- cleanup css, remove rules that no longer belong here
because they are now in either of jupyterlab-{courselevels,hidecell}

- workaround 16: revert to `defer` for `windowingMode` in the notebook settings,
unless the local setting 'windowing_mode_defer' is set to `false`

- splitcell-to-gridwidth is careful to avoid duplicate tags

- fix 14

- update README-use-case to show action on multiple cells

- actions apply on all selected cells, not just the active one
- more helpful readme with a gif micro demo