Renamed messagedlg to dialog, and added a couple of new dialogs -

SimpleMessageDialog, which just displays a plain message, and
TimeoutDialog, which shows said message, and disappears after a
timeout. ProcessingDialog now inherits form SimpleMessageDialog.

Another dialog, TextEditDialog, shows an editable/selectable piece of
text. This is used by a new CanvasPanel action, showCommandLineArgs,
which pops up a dialog allowing the user to copy the command line to
recreate the currently visible scene.