(vs. uses of sal_uInt16 representing such items' IDs). APPEND and ITEM_NOTFOUND
are members of ToolBox now (and have been bumped to the corresponding type's
max). The payload of VclEventIds
has been changed too (note that reading/writing of those payloads isn't very
consistent, though: for some of these, values are written that are never read,
while for some others no values are ever written but the reading code would like
to receive some value if the payload wasn't a nullptr).