void Renumerate
Assign consecutive numbers to individual items.
Result: stored in self (calling object)
Remarks:
Assign consecutive numbers starting with 0 to every items ItemNumber property. This numbers are assigned when the Items are added automatically. However, if the items in the list are moved or deleted, renumbering is neccessary, if there are parts of code that depend upon ItemNumber property and ItemNumber must coincide with the Index in the list at which this component is located.