OSX file dialog extensions (#2592)

Re-introduce OpenSavePanelDelegate for filtering when wildcard is provided (Spotlight search field was not working correctly, if extension was not known)
(and on macOS 10.11 allow programmatically showing the extra panel)

see http://www.github.com/wxWidgets/wxWidgets/pull/2592 and https://trac.wxwidgets.org/ticket/19324

co-authered-by: Jeff Young <jeff at rokeby dot ie>

include/wx/osx/filedlg.h

@@ -95,6 +95,7 @@ protected:
     bool m_useFileTypeFilter;
     int m_firstFileTypeFilter;
     wxArrayString m_currentExtensions;
+    WX_NSObject m_delegate;
 #endif
 
 private: