[DMN Designer] Hit Policy Collect does not need an Aggregation function

Page 86 of the OMG DMN1.1 Specification states "...if the aggregation attribute is not specified, the value of the decision table is the unordered set itself;"

Acceptance test

1. After saving and reopening is user able to achieve Collect hit policy
   1. with aggregating function
   2. without aggregating function
2. After saving and reopening is user able to change
   1. From any collect hit policy to non collect hit policy
   2. From non collect hit policy to collect hit policy
3. Change of hit policy is reflected in dmn file
4. Change of hit policy will show aggregation function just if Collect hit policy, otherwise None