Allow Time-Shift during Study Preview

[ja-fra]

As a further enhancement for #241 we allow to "fast-forward" the preview (change the offset while running in preview mode)

To further enhance the preview-mode of a study it shall be possible to adjust the date-offset while running a study in preview mode. This will allow to also test interventions based on previously recorded data.
As an example, it shall be possible to start the study preview on day 5, record data using observations and then "fast-forward" to day 16 with a conditional trigger based on the data recorded earlier.

At the time being, it is not yet decided if shall be possible to adjust the offset "backwards" (e.g. from day 16 rewind to back to day 7).