1. Added m_count and maintain its value in InsertItem, RemoveItem

1. Added m_count and maintain its value in InsertItem, RemoveItem
   etc. and return it from GetItemCount to save so many calls to
   ListView_GetItemCount.

2. If EVT_LIST_DELETE_ALL_ITEMS was not handled then the
   post-processing code to prevent LVN_DELETEITEM messages was not
   getting executed.  Changed it so the post processing always
   happens.

3. It's possible with some versions of comctl32 that garbage values
   can be passed in nmLV->lParam so don't try to get the internal item
   data pointer from it.  This should close bug# 659939

4. Added FreeAllInternalData from DeleteAllItems, (part of patch
   #672065) and also guard against processing LVN_CHANG* messages
   while freeing the internal data.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@19021 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775