Improve section editing for Flamingo

In Colibri the headings had a bottom-border, so the right floated section-editing icon was related to the Heading, see colibri.png

In Flamingo we changed the style, removing the border. We could try to improve the style in order to related the editing icon to the heading.

A first solution would be to change the icon from custom (resources/icons/xwiki/edit%2Dsection.png) to FontAwesome.
Another improvement could be to put it near the title, instead of all the way in the right, see flamingo_current_vs_proposal.png