Make wxDataViewCtrlBase::CreateDataObject() static

This function doesn't need to be virtual as it's not meant to be
overridden in, but just called from the derived classes. In fact, it
doesn't even have to be a member function at all, but leave it in the
class for now to avoid having to create a new header just for it, but
make it static.

Also add a comment explaining what is it for and what it does.

src/common/datavcmn.cpp

@@ -1690,6 +1690,7 @@ void wxDataViewCtrlBase::StartEditor(const wxDataViewItem& item, unsigned int co
 
 #if wxUSE_DRAG_AND_DROP
 
+/* static */
 wxDataObject* wxDataViewCtrlBase::CreateDataObject(const wxVector<wxDataFormat>& formats)
 {
     if (formats.empty())