Called to copy a region of the editor into the clipboard. This method is provided so that it can be overriden by subclasses. Do not call this method directly; instead, call copy.
Copy the data from start to end, extending the current clipboard contexnts if extend? is not #f.
See Cut and Paste for a discussion of the time argument. If time is outside the platform-specific range of times, an exn:application:mismatch exception is raised.