This addresses the following issues with vtkOutputWindow.

1. `vtkOutputWindow::DisplayText` method gets called for all types of
   messages and fired vtkCommand::MessageEvent. With no ability to
   determine the type of message in DisplayText overrides or
   MessageEvent handlers, this has little utility and one always ended
   up handling/overriding other `Display.*` methods. There was still a
   problem: it was challenging to determine if `DisplayText` was
   directly being called for output messages rather than via any of the
   other `Display.*` methods.

   This is fixed by adding `vtkOutputWindow::GetCurrentMessageType`
   protected method. Subclasses can use that in
   `vtkOutputWindow::DisplayText` overrides to determine message type.

   Additionally, a new `vtkCommand::TextEvent` is fired by
   `vtkOutputWindow::DisplayText` when the method is directly invoked.

2. vtkOutputWindow presented all messages to `cerr`. It should put
   `vtkOutputWindow::DisplayText` to `cout` as those are not exceptions.
   Added `UseStdErrorForAllMessages` (defaults to same behavior as
   before) to change this.