BRadioButton: Fix spurious Invoke()

MouseUp(): Don't call SetValue() and Invoke() when the button was
already the selected one.
BLooper: Add DispatchExternalMessage()

* This is primarily a service method for ports of widget tool kits
  that require single-threaded GUI. DispatchExternalMessage() calls
  DispatchMessage(), but also sets fLastMessage, so that
  [Detach]CurrentMessage() work correctly. This allows to detach a
  message in DispatchMessage() when called from the window thread,
  add it to a global queue, and later process the queued messages in
  a different thread that calls DispatchExternalMessage().
* BLooper/BWindow: Make sure fLastMessage is accessed only when locked.