Support mode transitions and dialog re-opening

[cxw42]

The dialog opens on a hotkey, similar to Esc entering normal mode. Add support for staying in normal mode where appropriate, i.e., reopening the dialog ready to accept a new command after the previous command has finished. Similarly, consider the dialog-closed state to be a hybrid of insert, replace, and visual modes. That way you will be able to use Word almost as Vim - i c r &c. to close the dialog and insert text, and <hotkey> to re-enter normal mode.