many wxItemContainer-related changes:

many wxItemContainer-related changes:

1. the main function for item insertion is now DoInsertItems() which allows
   for much more efficient addition of many new items at once
2. the items client data management is done entirely in wxItemContainer
   itself, the derived classes don't have to distinguish between void and
   object client data
3. many fixes for sorted controls, in particular implemented wxCB_SORT support
   in wxGTK combobox

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@47730 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775