1. perform deep comparison for the classes for which it makes sense in the
ports where this wasn't done yet
2. remove (shallow) comparison operators for the classes for which it does
not make sense (such as wxBitmap)
3. makes wxBitmap use COW on all ports
4. adds wxObject::IsRefTo()
5. centralizes and improves COW docs
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@42752 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
