Ensure that wxListEvent::m_itemIndex and m_item.m_itemId are in sync.

Ensure that wxListEvent::m_itemIndex and m_item.m_itemId are in sync.

Unfortunately we have two ways to get the item affected by wxListEvent: via
its m_itemIndex (or GetIndex()) and m_item.m_itemId (or GetItem()). This can't
be changed for the usual backwards compatibility reasons but at least make
sure that both contain the same values in the events sent by the generic
version (MSW one already did it).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70272 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775