Improvments about Gantt

[gdebrion]

Hello,

I noticed an error on Firefox and Chrome with the selectors #${id}-highlight. CSS selectors can not start with a digit, so I just reversed the name.

[gdebrion]

Sorry I didn't know that all commits to my master branch will be added to this merge request.
So, some improvments:

• The tooltip was not centered correctly on Firefox ;
• There is a typo in the call of upper_text function ;
• Someone add code to add shortened months but forgot that everybody doesn't want english 😄

[safwansamsudeen]

Thank you! Some of these are fixed in #423. Having popups at the place of entry is intentional.

What does the last commit do?