Fix TLW appearance in macOS 10.14's dark mode

Set toplevel window's background colour to the correct value of
NSColor.windowBackgroundColor (i.e. wxSYS_COLOUR_APPWORKSPACE). Without this,
the window's frame would be rendered slightly incorrectly, because of the way
the dark mode blends colors - even when no part of the window's interior, where
the background color would be used, is visible.

src/osx/nonownedwnd_osx.cpp

@@ -152,7 +152,7 @@ bool wxNonOwnedWindow::Create(wxWindow *parent,
     wxWindowCreateEvent event(this);
     HandleWindowEvent(event);
 
-    SetBackgroundColour(wxSystemSettings::GetColour( wxSYS_COLOUR_3DFACE ));
+    SetBackgroundColour(wxSystemSettings::GetColour( wxSYS_COLOUR_APPWORKSPACE ));
 
     if ( parent )
         parent->AddChild(this);