Initialize wxListItem::m_itemId to -1, not 0.

Initialize wxListItem::m_itemId to -1, not 0.

Currently we have two redundant pieces of data in wxListEvent: m_itemIndex and
m_item.m_itemId. The former is initialized to -1 by default which makes sense
as it means "no item". But the latter was 0 by default which not only didn't
make sense but also made it inconsistent with m_itemIndex for the events which
really didn't have any associated item.

Fix this by initializing m_itemId to -1 as well.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70270 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775