New option added to 'Hide scrollbars while message is displayed'

I wanted to hide scrollbars while message popup is displayed. This cannot be done easily from the template file as this is linked to popup apparition / dis-apparition.

Therefore I did implement a new option in the settings : 'Hide document scrollbars while message is displayed'
If checked, the javascript will take it into account.

Js, module and inc files have been modified.

By the way, I also corrected two typing errors (better-messeges-processed -> better-messages-processed).

Please find attached the modified module (I don't know how to use CVS, so please integrate it into the main branch as possible).

Best regards from France ;-)
Edouard