[Planner Dev] Duration to work convertion

[Xavier Ordoquy <xordoquy wanadoo fr>]

Hi,

I attached a patch to #134635
I think I could commit it but I'm not sure about the regression, esp for
fixed duration tasks.
However, it fixes the bugs for changing the tasks in the gantt view with
the mouse and an assignment different from 100%

Richard/Mickael, is it ok to commit ?

Regards,
Xavier Ordoquy.