Chart: show resources even if on leave during the period

[PierreRaybaut]

When resources are on leave during the period, chart does not show them at all, which is quite unexpected.
In other words, leave periods should be considered like other tasks for selecting the resources to show on a given chart.

[dylms]

Proposed fix :

• for resource chart, count resource if his task is in the period, even if he's on leave.
• for task chart, when processing task draw, create temp "tasks" on resources' leaves periods and print them above the task.
  May draw resources' leaves on another level according to Unify Chart Colors : tasks, dates, leaves, non-working days... #43 enhancement.