wxFD_PREVIEW implementation for wxGTK (slightly modified patch 1520690)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@43580 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/clipbrd.cpp

@@ -537,7 +537,9 @@ wxClipboard::~wxClipboard()
 {
     if ( m_lastDataObject )
     {
-        Clear();
+        // Retain any remaining clipboard data
+        // after application exits.
+        Flush();
     }
 
 #if wxUSE_OLE_CLIPBOARD