Don't check for model in wxDataViewCtrl::InsertColumn() in wxOSX.

This isn't done in neither generic nor the GTK versions and prevents from
inserting the columns into the control before associating a model with it.

Closes #16257.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76647 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Document that wxCANCEL_DEFAULT doesn't work in wxOSX/Cocoa.

See #16268.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76648 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775