put-file in editor:basic-mixin

Called when the user must be queried for a filename to save an editor. Starting directory and default name strings are passed in, but either may be #f to indicate that any directory is fine or there is no default name.

• (send an-editor:basic-mixin put-file directory default-name) -> string
     directory : string or #f
   default-name : string or #f

  Uses finder:put-file to find a filename. Also, sets the parameter finder:dialog-parent-parameter to the result of get-top-level-window.