Styling: Substitute “Table of contents”-caption with a proper icon to reclaim the space

[sjorek]

… well as the headline says; I'd like to substitute the “Table of contents”-caption in the toolbar with a proper icon, to reclaim the space the icon frees for the search-input-field.

A solution has already been implemented in https://github.com/sjorek/groc/tree/feature-collapsible-columns and can be viewed here: http://sjorek.github.io/groc/ But this solution has been mixed with another feature, related to https://github.com/nevir/groc/issues/112#issuecomment-24683929 .