[IMP] base: extract gantt validation

All views defined in the 'enterprise' addons have their validation done
manually in their respective addons. Each one ? No, one rebel still
resists to the standart way to work.
This commit moves the gantt validation to put it in its module. The idea
is (for developper) to have only one module to change when adding feature
in gantt view.

Task-2050019