get-file in editor:basic-mixin

Called when the user must be queried for a filename to load an editor. A starting directory string is passed in, but is may be #f to indicate that any directory is fine.

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

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