Merge: hightlight: improve infrastructure

infoboxes that popup where broken some time ago. This PR fix them and add two toggle for clients.

* `show_messages` to underline tokens and show error messages (or not)
* `show_infobox` to attach (or not) popupable infoboxes (those that where broken).

demo is available http://info.uqam.ca/~privat/tmp/nitlight/

Note: the UI is still terrible, but I have no better idea than these ugly popups to present complex contextual information.

Pull-Request: #2180
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>

diff --cc src/highlight.nit
Simple merge