Improve tabbed pane: allow right aligned tabs, allow tab groups

[deining]

This PR improves and extends the shortcodes for tabbed panes:

• the shortcodes learned to show right aligned tabs (by giving right=true on the tabpaneshortcode).
• the shortcodes now support spllitting up tabs in a left aligned and a right aligned tab group (by giving right=true on the dividing tab shortcode(s)). If wanted, even multiple tab groups can be rendered.

The PR also contains fixes as well as css and code improvements.
The PR is ready for review and subsequent merging.

[chalin]

Thanks @deining - can you rebase and address the build failures? Also, can you add a link to a page where we can preview the effects of the change?

[deining]

Thanks @deining - can you rebase and address the build failures?

Done and done.

Also, can you add a link to a page where we can preview the effects of the change?

Please have a look at this preview.

[chalin]

Wonderful! Thanks for this update (and your patience in getting it reviewed)!