set-message in name-message%

Sets the label for the control.

• (send a-name-message set-message file-name? msg) -> void
     file-name? : boolean
     msg : path string

  If file-name? is #t, msg is treated like a pathname, and a click on the name-message control creates a popup menu to open a get-file dialog.

  If file-name? is #f, msg is treated as a label string. Clicking on the name-message control pops up a dialog saying that there is no file name until the file is saved.