Use DPI Aware wxGetSystemMetrics

If no wxWindow is known, use wxTheApp->GetTopWindow().
Also use a wxWindow for all wxSystemSettings::GetMetric calls.

src/common/dlgcmn.cpp

@@ -210,7 +210,7 @@ wxSizer *wxDialogBase::CreateTextSizer(const wxString& message,
     const bool is_pda = wxSystemSettings::GetScreenType() <= wxSYS_SCREEN_PDA;
     if (is_pda)
     {
-        widthMax = wxSystemSettings::GetMetric( wxSYS_SCREEN_X ) - 25;
+        widthMax = wxSystemSettings::GetMetric( wxSYS_SCREEN_X, this ) - 25;
     }
 
     return wrapper.CreateSizer(message, widthMax);