message: Fix broken logic for detecting use of MSIE
  It's not immediately clear why the old code didn't work correctly, but
  it resulted in empty browser window sometimes (presumably when Edge
  wasn't available, although some user reports indicated otherwise).
  Use wxWebView::IsBackendAvailable() instead, which appears to do what we
  need reliably.
diff: @@ -139,13 +139,16 @@ FileViewer::FileViewer(wxWindow*)
  sizer->Add(new wxStaticLine(panel, wxID_ANY, wxDefaultPosition, wxSize(-1, 1)), wxSizerFlags().Expand().Border(wxLEFT|wxRIGHT, PX(5)));
  #ifdef __WXMSW__
  + if (wxWebView::IsBackendAvailable(wxWebViewBackendEdge))
  + {
  m_usesMSIE = false;
  - m_content = wxWebView::New(panel, wxID_ANY, wxWebViewDefaultURLStr, wxDefaultPosition, wxDefaultSize, wxWebViewBackendEdge);
  - if (!m_content)
  + m_content = wxWebView::New(panel, wxID_ANY);
  + }
  + else
  {
  + m_usesMSIE = true;
  wxWebViewIE::MSWSetEmulationLevel(wxWEBVIEWIE_EMU_IE11);
  m_content = wxWebView::New(panel, wxID_ANY);
  - m_usesMSIE = true;
  }
  #else
  m_content = wxWebView::New(panel, wxID_ANY);
  