message: Fix flickering attention bar on Windows
  It would be initially shown covering the entire window, then
  downscaled, causing visible flicker.
diff: #include <wx/statbmp.h>
  #include <wx/config.h>
  #include <wx/dcclient.h>
  +#include <wx/wupdlock.h>
  #ifdef __WXOSX__
  #include "macos_helpers.h"
  @@ -199,7 +200,8 @@ void AttentionBar::ShowMessage(const AttentionMessage& msg)
  // we need to size the control correctly _and_ lay out the controls if this
  // is the first time it's being shown, otherwise we can get garbled look:
  - SetSize(GetParent()->GetClientSize().x, GetBestSize().y);
  + wxWindowUpdateLocker lock(this);
  + SetSize(GetParent()->GetClientSize().x, 1);
  Layout();
  Refresh();
  