Support contains queries in SimpleEventQuery

The SimpleEventQuery class currently allows searching for events that have a property that starts or ends with a certain string, but it doesn’t allow searching for events that have a property that contains a certain string. A method contains(String) should be added to SimpleEventQuery with a corresponding CONTAINS CompareType in CompareQueryCondition to support this. The implementation would be very similar to commit 19a49eda that added the startsWith/endsWith methods. See also the corresponding proposal in the forum.