Update GTK clipboard handling

- Store the clipboard even if it was set in a GTK dialog.
- Fix a GtkTargetList leak.
- Notify GTK that the data is no longer available for
clipboard_get_cb(), so that GTK will no longer advertise nor attempt to
store the data.

widget/gtk/nsClipboard.h

@@ -39,13 +39,12 @@ private:
     static GdkAtom               GetSelectionAtom (int32_t aWhichClipboard);
     static GtkSelectionData     *GetTargets       (GdkAtom aWhichClipboard);
 
-    // Save global clipboard content to gtk
-    nsresult                     Store            (void);
-
     // Get our hands on the correct transferable, given a specific
     // clipboard
     nsITransferable             *GetTransferable  (int32_t aWhichClipboard);
 
+    void ClearTransferable(int32_t aWhichClipboard);
+
     // Hang on to our owners and transferables so we can transfer data
     // when asked.
     nsCOMPtr<nsIClipboardOwner>  mSelectionOwner;