It shouldn't be possible to add a "leave" task under a global task

[Aurelien2312]

The drop-down menu allows user to create a "leave/absence" task under a global task not associated to a ressource.
This leads to an exception when saving the planning.

Here is the stacktrace of the error (.PyPlanning_traceback.1.log)

[dylms]

A condition check if resource is in a task is already performed but setEnabled(False) on the "add leave" item mysteriously doesn't work on this particular action.
Fix found : hide instead of disable menu entry for adding leave if no resource in task