This is going to be fun, not.
Might be a good time to bypass gtk completely: #812.
Done in r16892 with one important caveat: we can't update the clipboard contents on the
SelectionData object we're given with Gtk.SelectionData.set (no idea why), so we update the clipboard and claim ownership of it. (we own the token)
This only works for text contents..
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1589