Implemented clear all children functionality in TreeViewControl. Reused to delete all sub folders and calculations from the root calculation group. Have CustomItemsOnlyContextMenuBuilder always add an item (also for standardized items), so that the same amount of items are added as when using a real implementation of the builder. Related to Issue [WTI-587] git-svn-id: https://repos.deltares.nl/repos/WettelijkToetsInstrumentarium/trunk@5908 5a3db67b-de53-47b5-99c8-a1c30a6650e2