Main additions:
- now it's possible to change the DVC style dynamically
- make models more robust against deleting items, and against other allowed user actions
- comment portions of the code to make it more didactive
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@59323 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775