erp5_calendar: Make Leave Request more user-friendly in parameterisation and appearance.

Follow a simple / non-simple approach, like for Task and its lines.

In default case, user would need only one Leave Request Period, which can be edited directly on LeaveRequest_view. So, in this case, hide the Listbox containing the Leave Request Periods and have default_leave_request_period being created / edited on background, which will acquire quantity, resource, start_date and stop_date from what is set at the view of the Parent.

In case one or more Leave Request Period are added manually, user would get listbox with sub-objects, and editing should be done directly on them, i.e. fields like my_default_leave_request_period_start_date will be disabled from LeaveRequest_view.

Additionally, now Leave Request has it's own:

• start_date: equal to the minimum start_date of all Leave Request Periods
• stop_date: equal to the maximum stop_date of all Leave Request Periods

These values are set by interaction on Leave Request Period, triggered on _setStartDate, _setStopDate. This helps so that leave_request_module listbox can have sortable dates.

Commit contains also test that should make it easier to understand.

Test results here

@seb we have talked on this and @cedric.leninivin I know you dealt with Leave Requests. Who else should be notified?