[FIX] project: don't use planned_hours in project gantt

The gantt view does not have enough data to properly display a project's length
based on only the planned hours.
It also makes it impossible to change the project's length using drag & drop.
It's safer to simply display the start and end dates recorded in the project
Fixes #2632
Merge remote-tracking branch 'odoo/7.0' into 7.0