Inserts an image into the editor.
The content of an editor can be changed by the system in response to other method calls, and such changes do not go through this method; use on-insert in text% or on-insert in pasteboard% to monitor content additions changes.
If filename is #f, then the user is queried for a filename. The kind must one of the symbols that can be passed to load-file.
After the filename has been determined, an image is created by calling on-new-image-snip. See also image-snip%.