one copy causes multiple gtk_clipboard_set_with_data

Hi,

the clipboard manager in sugar has encountered a problem when copying
text from abi: http://dev.laptop.org/ticket/716

Reading some code found that at every copy several addData would be
called and each of them would call gtk_clipboard_set_with_data.

As a result of it, several taking of ownership happen per copy, so for a
clipboard manager listening to owner-change it would appear as several
copies.

The patch attached mimics what is done in win32, when next to the
addData calls a closeClipboard is issued, for effectively putting the
data in the clipboard.

Have done some testing and it works. Could someone look at the patch and
comment on it?

Thanks,

Tomeu