save in frame:editor<%>

Saves the file being edited, possibly calling save-as if the editor has no filename yet.

• (send a-frame:editor save format) -> boolean
     format = 'same : (union 'guess 'standard 'text 'text-force-cr 'same 'copy)

  Returns #f if the user cancels this operation (only possible when the file has not been saved before and the user is prompted for a new filename) and returns #t if not.