Werk #1491: Fixed bug in dynamic lists where removing an item was not always possible

We use the so called valuespec ListOf() in several places where a dynamic amount of elements can be added to a list of things, for example the number of exceptions of a timeperiod in the timeperiod edit dialog.

Now, when you created a new timeperiod, added one exception and imediately removed it, you had no chance to save this timeperiod without adding another exception to the list. This has been fixed now.

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