Added pseudo-asynchronous clipboard implemenation to all ports excepts for GTK+

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@57510 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/clipbrd.h

@@ -60,6 +60,9 @@ public:
     // ask if data in correct format is available
     virtual bool IsSupported( const wxDataFormat& format ) = 0;
 
+    // ask if data in correct format is available
+    virtual bool IsSupportedAsync( wxEvtHandler *sink );
+    
     // fill data with data on the clipboard (if available)
     virtual bool GetData( wxDataObject& data ) = 0;