SyncControl: Fixed updating of data sources a sync control is restricted to

Before, the dropdown menu showing the data sets linked to a sync control wasn't updated properly. This gets fixed by adding a addDataSourceToControl method, that does exactly this.

Furthermore, added a "best guess" heuristic, which is executed for the first data set added to a sync control. It tries to guess whether the data should be synchronized using only valid time or both init and valid times. It checks whether for each valid time, there is a corresponding init time available. Then, both times are integrated, else only valid time.

Closes #569 (closed)