Purpose
=======
We should add some real timezone support for the global leaves
(resource.calendar.leaves without a resource assigned) currently it is
declared in UTC and never changed to the user or employee timezone.
Specification
=============
In order to support this, we have to add some choices for the user
defining the global leave. He can specify how the timezone is
configured,
- no value is specified, we keep the dates in UTC then convert those to
user timezone
- current user timezone specified, we set the timezone of the leave to
the company timezone
- a timezone is specified, we keep this specific timezone
RomainLibert
bb02b93c3a