"Point-in-Time" Preview Mode for Studies

[ja-fra]

As a further enhancement for #224 we add a basic "time-shift"-option for the preview-mode of a study:

When starting the preview of a study, the user can specify an offset (to day-level precision) for all date-calculations within the study.
As an example, for a study running over 3 Months, the user can specify to run the preview from day 60. As a consequence, all observations, interventions, etc... that would have happened within the first 59 days of the study will be skipped/ignored.
This allows to test and verify the configuration of a study towards the end of a study.

The Offset is specified when launching the preview mode and cannot be changed afterwards.