Allow configuring visible focus on Mac

Add wxWindow::EnableVisibleFocus() for changing focus ring behavior on
macOS (currently not implemented elsewhere, although GTK+ has a
discouraged option to do it).

src/osx/window_osx.cpp

@@ -2171,6 +2171,11 @@ bool wxWindowMac::AcceptsFocus() const
         return GetPeer()->CanFocus();
 }
 
+void wxWindowMac::EnableVisibleFocus(bool enabled)
+{
+    GetPeer()->EnableFocusRing(enabled);
+}
+
 void wxWindowMac::MacSuperChangedPosition()
 {
     // only window-absolute structures have to be moved i.e. controls